The Labour leader was hoping for a meeting with President Obama during his trip on Monday for their first White House encounter in his official capacity as leader of the opposition.
But on Saturday morning senior Labour figures said they did not know if the visit was still on given the diplomatic focus on the downing of passenger plane MH17 with 298 deaths – including 10 British citizens.
One aide said that the visit still seemed to be “more on than off” on balance.
On Saturday afternoon Mr Miliband’s team was confident that the visit would go ahead as planned. But others predicted that Mr Miliband could have to remain in London to attend a Commons debate about the tragedy.
Whitehall sources said there would be a statement to the Commons on Monday by a cabinet minister about MH17. It is not yet clear whether that will be by Mr Cameron, the prime minister, or Philip Hammond, the new foreign secretary.
On Saturday morning Mr Miliband called on Europe to work with the US to send a “clear message” to President Putin to stop the flow of arms and military support to Russian separatists in eastern Ukraine.
Speaking at Labour’s national policy forum in Milton Keynes, a major staging post in the party’s manifesto process, he began his speech with his thoughts on Ukraine and the Middle East.
“The tragic and horrifying end of flight MH17 has shocked people across the world,” he said. “We mourn the 10 British victims and all those who were lost . . . l know this could have been any of us or any of our loved ones.”
The opposition leader said that those responsible “must be brought to justice”. Europe had a clear responsibility, he said: “To work with the US to send a clear message to President Putin to stop the flow of arms and military support to Russian separatists and respect the sovereignty of Ukraine.”
Mr Miliband had expected a drop-in meeting with the US president on Monday along the lines that President George W Bush gave to David Cameron in 2008 when he was leader of the Tory opposition party.
In 2008 Mr Cameron was enjoying a double-digit lead in the opinion polls, amid public expectations that he was on track to enter Downing Street.
Mr Miliband, whose poll lead is more slender, is likewise the bookies’ favourite to win next year’s general election: but the contest is expected to be the closest political battle in Britain for a quarter-century.
The Labour leader is supposed to fly out to the US with key aides and Douglas Alexander, shadow foreign secretary. He is also scheduled to make a speech at the Centre for American Progress, the Washington think-tank.
Visits by opposition leaders – and prime ministers – to Washington represent a tightrope given the potential for a loss of face. Gordon Brown, even as the occupant in 10 Downing St, had to endure a humiliating five-minute conversation with President Obama in the White Hous kitchen.
Neil Kinnock’s attempt as Labour leader to project his qualities as a statesman were rebuffed by US Republican president Ronald Reagan in a famous White House snub: he was given less than half an hour to chat.
Historically the Labour party has been a philosophical fellow traveller with the US Democrat party, although President Obama has enjoyed warm relations with Mr Cameron during the current Parliament.
There is also a personal link: David Axelrod, a former senior aide to the president, is now providing advice to Labour ahead of the general election on a part-time basis.
Mr Miliband, an alumnus of Harvard, has not carried out many high-profile visits since winning the Labour leadership in late 2010: although he has been to Afghanistan, Israel and four European capitals: Paris, The Hague, Copenhagen and Stockholm.
A plan to go to India earlier this year was dropped amid widespread flooding across parts of Britain, which dominated the domestic political agenda for weeks.
If the trip to Washington goes ahead Mr Miliband could face criticism over his role in effectively smothering US intervention in Syria last August after President Assad used chemical weapons against the local population.