U.S. military bases and personnel in Iraq are coming under increasing attack from mortar-fire and bombings, including a roadside bomb that killed two U.S. soldiers on a mission in the capital Sunday, as militants are suspected of attempting to influence a contentious decision by Iraqi officials on whether to request that U.S. forces remain in the country beyond an end-2011 deadline.

"Various extremist groups and illegal militias have said they will increase attacks against U.S. forces and they are trying to do that to claim credit for driving out our forces," said U.S. Army Maj. Gen. Jeffrey Buchanan, a spokesman for the U.S. forces in Iraq.

There were at least 162 attacks against U.S. forces in Iraq last month, up from 128 in March and 93 in February, according to foreign security company in Iraq that tracks the data. The surge in attacks last month coincided with a rash of American military, political and diplomatic visits to the country, as the U.S. presses Iraqi officials to decide whether to request for an extended U.S. troop presence ahead of a looming deadline, and as opponents of the U.S. presence in Iraq press for the forces to leave. U.S. troop casualties hit an 18-month high in April at 11.

It isn't clear who is behind the attacks, though Iraq's restive northern provinces, typically the stomping grounds of insurgent groups alleged to be part of al Qaeda in Iraq, continued to show the highest number of attacks. There was a slight increase in Baghdad and more than a two-fold increase in southern Basra and Missan provinces, which border Iran, often accused by Western officials of arming militia groups in Iraq.

Seventy-two of April's attacks were mortar and rocket attacks on military facilities and on the heavily fortified "green zone," which houses the Iraqi government and the U.S. and other Western embassies.