United States troops battled insurgents holed up in houses and driving explosives-laden vehicles in a second town near the Syrian border, killing 28 in an expansion of their two-day-old offensive chasing Al Qaeda fighters along the Euphrates River valley, the military said.
Al Qaeda in Iraq on Sunday claimed to have taken two Marines captive during the fighting and threatened to kill them within 24 hours unless all female Sunni detainees are released from US and Iraqi prisons in the country.
The US military said the claim appeared false.
"There are no indications that the Al Qaeda claims are true," Multinational Force West, the command in the region, said.
It said it was conducting checks "to verify that all Marines are accounted for".
Even as the fighting continued, political differences among Iraqi leaders deepened ahead of the crucial October 15 national vote on a new constitution.
Iraq's Kurdish President Jalal Talabani called on the Shiite prime minister to step down over accusations that he is monopolising power in the government and ignoring his Kurdish coalition partners' demands, a spokesman for Talabani's Patriotic Union of Kurdistan said.
The US military says Al Qaeda in Iraq, the country's most fearsome insurgent group, has turned the area near the border into a "sanctuary" and a way-station for foreign fighters entering from Syria.
In Karabilah, Marines clashed with insurgents who opened fire from a building Sunday in a firefight that killed eight militants, the military said.