The visit came a day after members of Prime Minister Ibrahim al-Jaafari's ruling Shia Alliance bloc broke ranks publicly and joined calls for him to step down to improve chances of ending political paralysis.

"The fact that we're going out to have these discussions with the Iraqi leadership is a sign of the urgency which we attach to a need for a government of national unity," Rice told reporters who travelled with the two ministers from Britain.

US and Iraqi officials say a unity government, more than three months after December's election, is vital to averting all-out war after five weeks of spiralling sectarian bloodshed.

Rice arrived in Baghdad with Straw, a day after visiting his home region in northwest England.

The two were due to hold talks with Jaafari, who refuses to stand aside, and other leaders including President Jalal Talabani, a British official said. US officials make little secret of doubts about Jaafari's ability to unite and lead Iraq.

Straw said: "We're committed to Iraq, but we need to see progress and that is in everybody's interest."

Their efforts follow a visit last month by leading US senators who urged Iraq's fractious politicians to bury their differences and reach agreement.

"This is in many ways a time of testing for the Iraqi nation and for the Iraqi people. They need a government that can act on their behalf in this time of testing," said Rice.

The move against Jaafari, declared publicly by one leader and echoed, anonymously, by others came as parties held their latest round of talks on Saturday on a grand coalition with Kurds and Sunnis, who are adamant in their rejection of Jaafari.

Although Iraqi officials said they had reached a deal on forming a national security committee designed to share out more responsibilities among Iraq's feuding sects, there were no tangible signs of progress on the personnel of a new government.

That cannot happen, officials say, until there is accord on a prime minister -- unlikely to happen while Jaafari remains.

US and British officials worry that the paralysis will widen a political vacuum and fuel violence that is becoming more complex as Iraqis wait for their first full-term government since the US and British invasion toppled Saddam Hussein in 2003.