Kirkuk, Iraq — American officials, regional leaders and residents are increasingly worried that this northern oil-rich city could develop into a third front in the country’s civil war just as additional US troops arrive in Baghdad and Al Anbar province as reinforcements for battles there.

Al Qaeda-linked fighters recently have surfaced here, launching a wave of lethal attacks, US and Iraqi officials say. The attacks come amid a rise in communal tensions in the months before a referendum on the status of the city and the surrounding province.