“We are concerned primarily about the training of Shia extremists. We think there are about 50 members of the Iranian Revolutionary Guards,” Major-General Rick Lynch, commander of US forces south of Baghdad, told reporters.

Lynch said there had been an increase in “indirect fire attacks” on US forces in his area of command and that rocket attacks were becoming “more accurate and more effective”.

Washington has accused Shia Muslim Iran of fomenting violence in Iraq through its support for Shia militias, especially in southern Iraq.

The US military also accuses Iran of supplying deadly roadside bombs, the biggest killers of US troops in Iraq, to Iraqi militias and has displayed caches of weapons it says are from Iran.

Iran denies the charges and blames the 2003 US-led invasion for the sectarian violence between majority Shias and minority Sunni Arabs that has killed tens of thousands.

The US military believes the Revolutionary Guards’ Quds force is behind the shipping of weapons into Iraq, including armour-piercing “explosively formed penetrators”.

At a second round of landmark US-Iran talks on Iraqi security in July, US ambassador Ryan Crocker accused Iran of stepping up its support for militias in Iraq.

Crocker also warned Teheran that its Quds operatives would not be safe in Iraq.