"Specifically because it comes after two major attacks that happened over the past six days. The latest attack was at Bagram air field, which is the biggest air field in the country," she said.

"Before that - just a day before - there was a suicide bombing here in Afghanistan where six foreign soldiers were killed.

"So certainly, the Taliban are trying to show that they are there and they are able to fight."

Taliban offensive

The attack on Bagram airbase, near Kabul, on Wednesday left one US contractor dead and nine Nato troops wounded.

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The Taliban claimed responsibility for that attack, saying 20 suicide bombers took part in the assault, which included rockets, small arms and grenades, Nato's International Security Assistance Force, or Isaf, said.

The recent spate of attacks follow the Taliban's announcement that it would launch a spring offensive against Nato and Afghan forces in response to Nato's plans for a military campaign on the group's southern stronghold of Kandahar.

Separately on Saturday, three foreign soldiers and one civilian working with Isaf were killed in two separate incidents in southern Afghanistan.

"In a separate incident, also in southern Afghanistan, another Isaf service member died following an improvised explosive device (IED) attack," Nato said in a statement on Saturday.

The latest casualties bring the number of foreign soldiers killed in Afghanistan this year to 215, according to figures on the icasualties.org website.

In 2009, the deadliest year since the US-led an invasion that overthrew the Taliban in late 2001, 520 foreign soldiers were killed.

The US and its Nato allies are increasing to 150,000 their military deployment in Afghanistan, about two-thirds of which is American.