The pressure on President George Bush over Iraq intensified yesterday, when two senior Republican senators put forward a bill challenging his Iraq strategy and setting out a route for withdrawal of US troops.

The draft bill by the Republicans, John Warner and Richard Lugar, proposes switching US troops from fighting the insurgency to a more narrow remit.

This role would involve protecting Iraq's borders, fighting only al-Qaida of Mesopotamia rather than the bigger nationalist and tribal insurgents, and protecting US bases and other interests in the country. The bill echoes one that has been put forward by the Democrats, which also includes a deadline for withdrawal of April 1 next year.

The Democrats have a majority in both the House and the Senate. But a simple majority is not sufficient in the 100-member Senate, and the Democrats have so far fallen short of the 60 votes they need to push through an anti-war bill.

Mr Bush now faces the dangerous prospect of an alliance between the Democrats and Republicans. Congress could still push through a bill in the last three weeks before the August recess. But the Republicans may prefer to wait until September to hear a progress report from the US commander in Iraq, General David Petraeus, and the US ambassador to Baghdad, Ryan Crocker.

Earlier this year the Democrats tried to push through legislation to begin the withdrawal of US troops or face a cut in funding for the war. But at the time it did not have Republican support.

But there has been a significant shift in recent weeks by the Republicans, who have become frustrated at the lack of progress in Iraq and are increasingly aware of public disenchantment with the war. Many of the Republican dissidents face re-election next year.

Both Mr Warner, who is on the Senate armed services committee, and Mr Lugar, who is on the foreign affairs committee, are respected figures within the Senate and are likely to attract the support of many other Republicans.

Their bill says: "Given continuing high levels of violence in Iraq and few manifestations of political compromise among Iraq's factions, the optimal outcome in Iraq of a unified, pluralist, democratic government that is able to police itself, protect its borders, and achieve economic development is not likely to be achieved in the near future."

The political manoeuvring in Washington was going on as US forces killed 13 people during a clash with Iraqi police in a Shia district of Baghdad. The shootout at a police checkpoint highlighted the extent of American distrust in the police force it helped establish. In the Bush administration's report on progress in Iraq, delivered to Congress on Thursday, the state of the Iraqi police was judged unsatisfactory.

The US military said in a statement that an Iraqi police lieutenant had been detained on suspicion of planning roadside bomb and mortar attacks on US forces and having links with Iranians allegedly stirring up violence.

In a separate incident, a New York Times reporter, Khalid Hassan, 23, was shot dead to his way to work in Baghdad. His death came fewer than 24 hours after two Reuters staff were killed in Baghdad.