Britain will maintain a garrison of 4,000 troops at Basra airport for the forseeable future, whatever the pressures on the armed forces, defence officials said last night. They described the British garrison as being there "for the long term" after talks in London yesterday between General David Petraeus, commander of US forces in Iraq, Des Browne, the defence secretary, and Sir Jock Stirrup, chief of the defence staff.

It was said that the troops were needed to continue their task of training Iraqi forces and also to maintain what officials called "political credence" with the US.

After an hour's meeting at Downing Street with Gordon Brown, Petraeus said British forces had been "invaluable" in providing intelligence, air and logistics support for Iraq troops engaged in the recent Charge of the Knights operation against Mahdi army militia elements supporting the Shia cleric Moqtada al-Sadr.

Asked how long the US wanted 4,000 British troops stationed outside Basra, Petraeus responded: "The answer right now is we don't know ... We need to work [it] out in the next month or two as we look at the so-called troop-to-task analysis."

British defence sources said that did not mean a decision would be taken then to reduce the number of UK troops in Iraq. The hope expressed by Gordon Brown late last year that the number could be reduced to 2,500 this spring clearly has been dashed. Petraeus said the immediate focus was on helping Iraqi forces in their operation against Mahdi army and criminal elements in Basra. He said more than 1,000 Iranian-made mortar and artillery rounds as well as rockets had been found in the city. British officials said that though private polls suggested the people of Basra were relatively optimistic about the future, the situation in Basra remained "fragile". About 800 US troops, including special forces, are helping a combined force of 30,000 Iraqi army and police. About 150 British troops are involved. The British and US military troops involved were advisers, officials said.

Petraeus, who was accompanied by Ryan Crocker, US ambassador to Iraq, said the supplies of Iranian weapons into Iraq were a "huge concern". British forces are not getting involved in these disputes in what ministers call their "overwatch" role, providing air support, including helicopter supplies and surveillance, to Iraqi forces in the city.

They are also training the Iraqi army's 14th division based in Basra. British military planners and commanders hoped that by now they could have substantially reduced the number of British troops in Iraq, and are frustrated by the time it has taken the Iraqi army to develop into an effective force. Petraeus admitted that Iraqi forces in the recent operation in Basra had got off to a "shaky start. Some were not equal to the task," he said.

Iraq's interior ministry said yesterday that more than 1,000 Iraqis had been killed in April, 968 of them civilians.