BAGHDAD, Wednesday (AFP) US troops may start withdrawing from Iraq next March, a US general said, as the new US ambassador to the volatile country pledged to help Iraqis crush a ruthless insurgency.

As many as four or five US military brigades (up to 20,000 people) could leave then if the country's ethnic groups can agree on a constitution and elect a government that has broad support, Lieutenant General John Vines said in Washington via video link from Iraq.

"I suspect we will probably draw down capability after the election, because Iraqi security forces are more capable," Vines said. He was referring to elections at the end of this year set to follow hoped-for approval of a new constitution in the autumn. The number-two US commander in Iraq predicted that the insurgency would dwindle rapidly if the political process were successful, but said any drawdown would depend on conditions on the ground.

In Britain, Foreign Secretary Jack Straw told lawmakers the US-led coalition did not have an "open-ended" commitment to staying in the country.

But when asked for reassurance that progress was being made in Iraq Straw said: "Violence is at an unacceptable level, higher than was anticipated."

Iraqi Foreign Minister Hoshyar Zebari said Iraq would not be surprised and would understand if the United States begins to start withdrawing troops next year.