Nearly 4,000 US soldiers have arrived in Baghdad while bombings and shootings killed 16 people Wednesday, including three Sunni brothers who were shot to death in Baghdad.

The party's most liberal members, especially in the House, say they will vote against money for continuing the war if there's no binding language on troop drawdowns. Bush and almost all congressional Republicans continue to insist on a spending bill with no strings attached on troop movements.

Bush on Tuesday rejected legislation pushed by Democratic leaders that would require the first US combat troops to be withdrawn by Oct. 1 with a goal of a complete pullout six months later.

"This is a prescription for chaos and confusion and we must not impose it on our troops," Bush said in a nationally broadcast statement from the White House. "It makes no sense to tell the enemy when you plan to start withdrawing."

The standoff gives Republicans leverage, because even with the liberals' votes, Democrats don't have enough support to override Bush's veto. It will force Speaker Nancy Pelosi, D-California, to seek more Republican help in drafting a new bill that Bush might accept, her allies and opponents say.

"I think the Democrats are in a box," Rep Eric Cantor, R-Virginia, said in an interview Tuesday. "We're pretty resolute on our side. We are not going to tie this funding to any type of withdrawal deadline or any type of redeployment deadline."

Some Democrats believe the GOP solidarity will crack over time, noting that polls show heavy public support for a withdrawal plan.

Lawmakers in both parties agree that a workable compromise is a huge challenge in the coming days or weeks. Because Democrats control the House and Senate, the pressure is mainly on them to craft a bill that Bush will sign, and thus avoid accusations that they failed to finance troops in a time of war.

Many Democrats say a new spending bill must include so-called benchmarks for progress in Iraq that, if not met, would trigger movements of US troops out of the country or perhaps to non-urban areas that see little sectarian violence. A new spending bill "has got to be tied to redeployment," said Rep Rahm Emanuel, D-Illinois, the House's fourth-ranking Democratic leader.

Emanuel conceded, however, that Democrats have yet to figure out where they will find the votes.

The situation frustrates Democrats, who won control of the House and Senate in an election that largely focused on Iraq.

Moreover, Democrats showed impressive solidarity in passing the bill that Bush vetoed Tuesday, losing only 14 House Democrats while holding 216. Top Democrats say they have no hope of replicating that showing once they begin making even modest concessions in response to Bush's veto.

That makes them dependent on Republican help to some degree perhaps a lot. As long as most GOP lawmakers stick with the president, "the question is how much policy and change we can push in Iraq," Emanuel said.

In his veto statement Tuesday, Bush again rejected the notion of an "artificial deadline" for troop withdrawals. But he added, "I'm confident that with good will on both sides we can agree on a bill that gets our troops the money and flexibility they need, as soon as possible."

Pelosi, who was to join Republican and Democratic leaders from both houses in a meeting with Bush on Wednesday, told reporters after Bush's remarks: "The president wants a blank check. The Congress is not going to give it to him."

Democrats will work with the White House, she said, "but there is great distance between us right now."

Numerous possible compromises are being floated on Capitol Hill, all involving some combination of benchmarks. Some would require Bush to certify monthly that the Iraqi government is fully cooperating with US efforts in several areas, such as giving troops the authority to pursue extremists. Others would require an Iraqi-run programme to disarm militias and a plan to distribute oil revenues fairly.

The key impasse in Congress is whether to require redeployments of US troops if the benchmarks are not met. Many Democrats insist on it, and many Republicans vow not to budge.

"Our members will not accept restraints on the military," House Minority Whip Roy Blunt of Missouri told reporters Tuesday. He suggested tying benchmarks to continued US nonmilitary aid to Iraq, an idea that many Democrats consider too weak.

Meanwhile, nearly 4,000 US soldiers have arrived in Baghdad as a crackdown aimed at quelling the sectarian violence enters its 12th week. Bombings and shootings killed 16 people Wednesday, including three Sunni brothers who were shot to death in Baghdad.

An international conference on Iraq begins Thursday in the Egyptian Red Sea resort of Sharm el-Sheik at which the US administration is expected to press hard for countries to forgive billions of dollars in Iraqi debt to help the Shia-led government as it tries to rein in the violence.

A bomb tore through a minibus travelling south from Baghdad to the town of Hilla through an area rife with insurgents on Wednesday, killing 11 passengers, local mayor Muayad Fadhil said.

Another three people were killed and 20 wounded when suspected insurgents fired a salvo of mortar shells into a residential area of his town, Mahmudiyah, 30km south of Baghdad security officials said.

Another mortar attack was launched against a primary school in the neighbouring town of Iskandiriyah, killing a young girl and injuring her teacher, said Lieutenant Kadhim Kashash of the town police.

In a separate attack, four people from the same family were injured in a bomb blast targeting a house of a policeman in the same area.

The US military said Wednesday that the fourth of five brigades being sent to help Iraqi security forces as part of the crackdown had arrived this week.

The 4th Brigade, 2nd Stryker Brigade Combat Team from Fort Lewis, Wash., which includes about 3,700 soldiers, will be deployed in the Baghdad area and in northern Iraq, the military said. Officials want the rest in place by June, for a total of 160,000.

On Tuesday, Bush rejected legislation pushed by Democratic leaders that would require the first US combat troops to be withdrawn by Oct. 1 with a goal of a complete pullout six months later. It was only the second veto of his presidency.

"This is a prescription for chaos and confusion and we must not impose it on our troops," Bush said in a nationally broadcast statement from the White House. He said the bill would "mandate a rigid and artificial deadline" for troop pullouts, and "it makes no sense to tell the enemy when you plan to start withdrawing."

Democrats accused Bush of ignoring Americans' desire to stop the war, which has claimed the lives of more than 3,350 members of the military.

Ismail Qassim, a 41-year-old Shia electricity ministry employee in Baghdad, welcomed the move.

"In spite of all the problems Iraq is facing because of the American presence, there is some need for them at least for one more year because of the sectarian strife in Iraq and corruption in the security services," he said.

But Sameer Hussein, a 22-year-old Sunni college student in Baghdad, said he wanted the US forces to withdraw but didn't think they ever would.

"Even if they will withdraw they will leave permanent military bases in Iraq and that is something Iraqi people will reject," he said.

A senior Interior Ministry official, meanwhile, said officials were trying to gain custody of Abu Ayyub al-Masri's body amid widespread scepticism over claims that the leader of al-Qaeda in Iraq had been killed.

Maj. Gen. Hussein Kamal declined to comment further, but a police official in Anbar province said al-Masri died when his explosives belt detonated during fighting but security forces could not retrieve the body because it was in a part of the desert controlled by the terror group. US authorities urged caution about the reports, saying they had not been confirmed and warning that even if the claim were true, the death of the shadowy Egyptian militant likely would not spell the end of the terror movement in Iraq.