The breakthrough measure, reached late Wednesday and likely to gather wide legislative support, means that the White House could face an embarrassing -- but ultimately symbolic -- vote of no confidence in its latest military plan.

The agreement was announced just hours after Democrat Nancy Pelosi, the speaker of the House of Representatives, met Bush at the White House and found "areas of agreement" on Afghanistan and Iraq.

And it follows days of increased rhetoric against Iran for its alleged involvement in supporting attacks on US troops in Iraq.

Some legislators fear the president is setting the groundwork for US military action against Iran, as the US aircraft carrier John Stennis heads to the Gulf to join the aircraft carrier Dwight D. Eisenhower, already in the region.

In the meantime, the unpopular war in Iraq "is perhaps the most vital issue facing our country," said Senator Susan Collins and one of the members of Bush's Republican Party, as she announced that a compromise resolution opposing the troop surge was reached.

"It is crucial that the Senate go on record expressing its opposition to the president's plan to send 21,500 troops to Iraq," she said.

The announcement was at odds with the warm feelings expressed after Pelosi met with Bush.

"It was a very constructive meeting," said Pelosi, a tough Bush critic. "It was respectful and we found some areas of agreement in terms of our sizing up what the challenges are there."

Pelosi, who led a bipartisan delegation of lawmakers just back from Afghanistan and Iraq, said they had briefed the president on their findings.

"We found a number of areas where we agree, there are some areas of discussion," said Republican Representative David Hobson, also at the meeting.

In Baghdad, Prime Minister Nuri al-Maliki, in an interview with CNN, warned the United States and Iran to take their quarrel elsewhere, saying he would not permit his battered country to be caught in the crossfire.

"There is a struggle between Iran and America, and we have told the Iranians and Americans, we know that you have a problem with each other," Maliki said, speaking through a translator.

"But we're asking you please, solve your problems outside of Iraq. We do not want the American forces to take Iraq as a field to attack Iran or Syria, and we will not accept Iran to use Iraq to attack the American forces."

Bush last week issued orders allowing US forces to capture or kill Iranian agents in operating in Iraq. Washington accuses Tehran of arming and training Shia militias.

"We will not let Iran play a role against the American army and not allow America to play a role against the Iranian army," Maliki said. "Everyone should respect the sovereignty of Iraq."

US defence officials meanwhile said they were investigating whether Iranian agents were involved in a January 20 raid on an Iraqi compound in Karbala where five US soldiers were killed.

A senior US defence official said the investigation was incomplete and no conclusions had been reached.

A second defence official confirmed that investigators were looking for an Iranian connection in part because the operation was a so sophisticated, involving an elaborate ruse to penetrate the compound's security.

The attack followed a US crackdown on Iranians operating in Iraq, including a US raid in the northern city of Irbil in which five Iranians were detained over protests by Tehran that they were members of its consular staff.

Meanwhile, at least nine people were killed in Baghdad Thursday as a bomb tore through a minibus in a predominantly Shia commercial district and mortars hit a Sunni area, officials said.

The bombing of the minibus killed at least six and wounded eight, police said. It struck at noon in the district of Karradah, which has been hit by several bombings in recent weeks as insurgents try to maximise the number of people killed before a planned US-security crackdown.

A suicide car bombing devastated the area less than a week ago, killing some 30 people.

Mortar rounds slammed into a Sunni neighbourhood in Baghdad for the third day in a row, killing at least three people and wounding 10, hospital officials said.

Five mortar rounds struck the streets of the northern area of Azamiyah on Thursday morning, according to officials who gave the casualty toll.

The Sunni area has been hit daily by mortar attacks since Tuesday, when at least five people were killed and 20 wounded in apparent retaliation for insurgent bombings against predominantly Shia areas coinciding with the Shia holy day of Ashoura.

Four people were killed in an afternoon mortar attack Wednesday in Azamiyah.

Elsewhere, the US military said that a soldier died Thursday of wounds sustained in fighting in Anbar province.

The soldier assigned to Multi National Force West was killed after being wounded in fighting on Tuesday in the Sunni insurgent stronghold west of Baghdad, the military said in a statement.

The soldier was not identified pending notification of relatives.