Susan Sontag, one of America's most influential intellectuals, internationally renowned for the passionate engagement and breadth of her critical intelligence and her ardent activism in the cause of human rights, died on Tuesday of leukaemia in a New York City hospital. She was 71.

The author of 17 books translated into 32 languages, she vaulted to public attention and critical acclaim in 1964 with "Notes on Camp", written for Partisan Review and included in Against Interpretation, her first collection of essays, published two years later.

Sontag wrote about subjects as diverse as pornography and photography, the aesthetics of silence and the aesthetics of fascism, puppet theatre and the choreography of Balanchine, as well as crafting portraits of such writers and intellectuals as Antonin Artaud, Walter Benjamin, Roland Barthes and Elias Canetti.

She was a fervent believer in the capacity of art to delight, to inform and to transform. "We live in a culture," she said, "in which intelligence is denied relevance altogether, in a search for radical innocence, or is defended as an instrument of authority and repression.

"In my view, the only intelligence worth defending is critical, dialectical, sceptical, desimplifying."

In a Rolling Stone article in 1979, Jonathan Cott called Sontag a writer who was "continually examining and testing out her notion that supposed oppositions like thinking and feeling, consciousness and sensuousness, morality and aesthetics can in fact simply be looked at as aspects of each other much like the pile on the velvet that, upon reversing one's touch, provides two textures and two ways of feeling, two shades and two ways of perceiving".

A self-described "besotted aesthete" and "obsessed moralist", Sontag sought to challenge conventional thinking.

"From the moment I met Susan Sontag in 1962, I felt myself to be in the presence of a woman of astonishing intelligence and the most exemplary literary passions," said the novelist Carlos Fuentes. "I admired her work and her life without reservation."

She was born on January 16 1933 in New York City and raised in Tucson and Los Angeles, the daughter of a schoolteacher mother and a fur trader father who died in China when Sontag was five.

She was a graduate of North Hollywood High School and attended the University of California, Berkeley, and the University of Chicago, which she entered when she was 16, and Harvard and Oxford.

In 1950, while at the Uni versity of Chicago, she met and 10 days later married Philip Rieff, a 28-year-old instructor in social theory. Two years later she had a son, David, who is now a writer. She divorced in 1959 and never remarried.

The first novel that affected her was Victor Hugo's Les Mise{'}rables.

"I sobbed and wailed and thought [books] were the greatest things," she recalled. "I discovered a lot of writers in the Modern Library editions, which were sold in a Hallmark card store, and I used up my allowance and would buy them all."

She remembered as a small girl lying in bed looking at her bookcase against the wall. "It was like looking at my 50 friends. A book was like stepping through a mirror. I could go somewhere else. Each one was a door to a whole kingdom."

Upon reading Jack London's Martin Eden, she determined she would become a writer. "I got through my childhood," she told the Paris Review, "in a delirium of literary exaltations."

She began to frequent the Pickwick bookstore on Hollywood Boulevard, where she went "every few days after school to read on my feet through some more of world literature, buying when I could, stealing when I dared".

She also became a "militant browser" of the international periodical and newspaper stand near Hollywood Boulevard and Highland Avenue, where she discovered the world of literary magazines.

At 26 she moved to New York City, where she taught the philosophy of religion at Columbia University.

At a cocktail party, she encountered William Phillips, one of Partisan Review's legendary founding editors, and asked him how one might write for the journal. He replied: "All you have to do is ask." "I'm asking," she said.

Soon Sontag's provocative essays on Albert Camus, Simone Weil, Jean-Luc

Godard, Kenneth Anger, Jasper Johns and even The Supremes began to spice Partisan Review's pages.

Sontag recoiled at what she regarded as the artificial boundaries separating one subject, or one art form, from another.

She devoted herself to demolishing "the distinction between thought and feeling which is really the basis of all anti-intellectual views: the heart and the head, thinking and feeling, fantasy and judgment. Thinking is a form of feeling; feeling is a form of thinking."

Her quest was admired by such writers as Elizabeth Hardwick, a founder of the New York Review of Books, whose editors quickly embraced Sontag.

In her introduction to A Susan Sontag Reader, Hardwick called her "an extraordinarily beautiful, expansive and unique talent".

Others were less impressed. John Simon accused Sontag of "a tendency to sprinkle complication into her writing" and of tossing off "high-sounding paradoxes without thinking through what, if anything, they mean".

In 1976, at 43, Sontag discovered she had advanced cancer in her breast, lymphatic system and leg. She was told she had a one-in-four chance of living for five more years. "My first reaction was terror and grief. But it's not altogether a bad experience to know you're going to die. The first thing is not to feel sorry for yourself," she said. She set about learning as much as possible about the condition. But after undergoing a radical mastectomy and chemotherapy, she was pronounced free of disease.

Sontag later wrote "Illness as Metaphor", an influential essay condemning the use of tuberculosis and cancer as metaphors that transfer responsibility for sickness to the victims, who are made to believe they have brought suffering on themselves. Illness, she insisted, is fact, not fate. Years later, she would extend the argument in the book-length essay AIDS and its Metaphors.

An early and passionate opponent of the Vietnam war, Sontag was both admired and reviled for her political convictions. In a 1967 Partisan Review symposium, she wrote that "America was founded on a genocide, on the unquestioned assumption of the right of white Europeans to exterminate a resident, technologically backward, coloured population in order to take over the continent".

In her rage and growing despair, she concluded that: "The truth is that Mozart, Pascal, Boolean algebra, Shakespeare, parliamentary government, baroque churches, Newton, the emancipation of women, Kant, Marx, Balanchine ballets, et al, don't redeem what this particular civilisation has wrought upon the world. The white race is the cancer of human history; it is the white race and it alone with its ideologies and inventions which eradicates autonomous civilisations wherever it spreads, which has upset the ecological balance of the planet, which now threatens the very existence of life itself."

Considering herself neither a journalist nor an activist, Sontag felt an obligation as "a citizen of the American empire" to accept an invitation to visit Hanoi at the height of the US bombing campaign in May 1968. A two-week visit resulted in a fervent essay seeking to explain Vietnamese resistance to American power. Critics excoriated her for what they regarded as a naive sentimentalisation of Vietnamese communism.

That same year, Sontag also visited Cuba, after which she wrote an essay for Ramparts magazine calling for a sympathetic understanding of the Cuban revolution. But two years later she joined the Peruvian novelist Mario Vargas Llosa and other writers in publicly protesting against the regime's harsh treatment of Heberto Padilla, one of Cuba's leading poets.

Ever the iconoclast, Sontag had a knack for annoying both the right and the left. In 1982, in a meeting in Town Hall in New York to protest against the suppression of the Solidarity movement in Poland, she declared that communism was fascism with a human face.

She was unsparing in her criticism of much of the left's refusal to take seriously the exiles and dissidents and murdered victims of Stalin's terror and the tyranny communism imposed wherever it had triumphed.

Ten years later, almost alone among American intellectuals, she called for vigorous western and American intervention in the Balkans to halt the siege of Sarajevo and to stop Serbian aggression in Bosnia and Kosovo.

Then, in the aftermath of the terrorist attacks of September 11 2001, Sontag offered a bold and singular perspective in The New Yorker: "Where is the acknowledgment that this was not a 'cowardly' attack on 'civilisation' or 'liberty' or 'humanity' or 'the free world' but an attack on the world's self-proclaimed superpower, undertaken as a consequence of specific American alliances and actions?"

Sontag had never been so public as she became over the next three years, publishing steadily, speaking constantly and receiving numerous international awards, including Israel's Jerusalem Prize, Spain's Prince of Asturias Award for the Arts and Germany's Friedenspreis (Peace Prize).

In an interview for the Paris Review, in 1995, Sontag was asked what she thought was the purpose of literature.

"A novel worth reading," she replied, "is an education of the heart. It enlarges your sense of human possibility, of what human nature is, of what happens in the world. It's a creator of inwardness."

She was the cartographer of her own literary explorations. Henry James once remarked: "Nothing is my last word on anything." For Sontag, as for James, there was always more to be said, more to be felt.

In addition to her son, she is survived by a sister, Judith Cohen.

Steve Wasserman The author is a staff writer with the Los Angeles Times. This obituary is published by arrangement with that newspaper