U.S. Secretary of State Condoleezza Rice says Iraqis are losing patience with their politicians for failing to form a unity government after nearly four months of negotiations.

Rice spoke Sunday, hours after she and British Foreign Secretary Jack Straw paid a surprise visit to Baghdad, in a push to break the political impasse.

The visit comes as political pressure mounts on Prime Minister Ibrahim al-Jaafari to step aside as the Shi'ite nominee for a second term. Kurdish and Sunni leaders say he has failed to stem Iraq's insurgency, and has not done enough to blunt rising sectarian bloodshed.

Senior figures in al-Jaafari's Shi'ite Alliance broke ranks with the prime minister on Saturday and called on him to quit.

Rice said she told Iraqi leaders that whoever becomes prime minister must have enough support to form a unity government quickly.

Meanwhile, a U.S. statement says a U.S. military helicopter lost Saturday south of Baghdad was shot down. The statement said the two pilots of the aircraft are presumed dead.

Separately, U.S. officials said two American soldiers were killed late Saturday by a roadside bomb in central Baghdad.

A third statement said four insurgents were killed and another wounded Saturday, in two failed attacks on coalition troops near Balad.