Mohammad Hanif Hanifi, the senior government official in Nirkh district in Wardak, said a US vehicle was struck by an IED in a village at about 6pm (1330GMT) - a time that forces routinely undertake patrols.

The second US patrol hit a device 15 to 20 minutes later, he said.

A helicopter subsequently arrived to evacuate casualties, Hanifi said.

In other incidents on Monday, four Afghan policemen were killed when the Taliban attacked a security post near the town of Kunduz in the north.

"Four policemen were martyred when Taliban attacked the police post," General Abdul Razaq, the Kunduz police chief, said.

Increasing casualties

About 3,000 US soldiers have moved into Wardak and Logar, two provinces which adjoin Kabul, in recent months in order to quell increasing opposition attacks before the August 20 elections.

US military officials said that they expected a 50 per cent rise in IED attacks this year in response to the increased troop numbers.

"We certainly do expect an increase and there has been an increase in insurgents' use of IEDs, which is their primary means of conducting attacks because they can't launch an effective direct attack," Greg Julian, the senior US spokesman in Afghanistan, said.

"And we anticipate a continued increase in the use of this tactic."

A total of 64 US personnel have now been killed this year, up from 34 soldiers in the first five months of last year.

After Monday's killings 121 international soldiers have died this year.

Isaf said that homemade IEDs cause 70-80 per cent of casualties to foreign troops in Afghanistan.

Fighters killed

But opposition groups also suffered losses according to the US military, who said in various announcements that they had killed 11 fighters in different air raids and clashes in Wardak and Ghazni in the east and in the central province of Uruzgan.

Violence in Afghanistan has increased during the past three years to its highest level last year since the 2001 US-led invasion.

US President Barack Obama has is sending more than 20,000 additional soldiers to the country to suppress attacks.

The US entered the country to end rule by the Taliban, whom they said were harbouring al-Qaeda fighters, in the wake of the September 11, 2001, attacks.