(Writes through)

By Akram Walizada

KABUL, March 13 (Reuters) - A suicide car bomber killed eight Afghan civilians in an attack on U.S. troops near the airport in Afghanistan's capital on Thursday, a NATO spokesman said.

Taliban Islamist militants have threatened to step up suicide attacks on the capital Kabul this year in a campaign to wear down the will of NATO countries to carry on the fight in Afghanistan and force a withdrawal of foreign troops.

Thirty-five civilians were also wounded in Thursday's attack, but the four U.S. soldiers inside the two vehicles targeted suffered only minor cuts and bruises.

The Taliban claimed responsibility.

"The foreign occupying forces and their Afghan slaves should get ready for the mujahideen's suicide and guerrilla attacks this summer. This new year will be the bloodiest," spokesman Zabihullah Mujahid said by telephone from an unknown location.

The Taliban and Afghan government and foreign forces are gearing up for an escalation in fighting after a harsh winter.

But NATO is struggling to come up with more troops with some European alliance members reluctant to let their forces fight in southern and eastern Afghanistan where U.S., British, Canadian and Dutch soldiers clash almost daily with Taliban militants.

Canada's parliament is expected to pass a motion on Thursday to extend the mission of its 2,500 troops in southern Afghanistan till 2011, but only on the condition that NATO allies come up with another 1,000 soldiers to reinforce its combat forces.

So far no country has come up with any concrete offer.

FRUSTRATION

There are currently just over 50,000 foreign troops in Afghanistan, compared to some 160,000 in Iraq, a country only two-thirds the size and with a smaller population.

The Afghan army currently has around 70,000 troops and NATO countries have been slow answering U.S. calls to provide more trainers and mentors to help expand the force.

Afghan Defence Ministry officials have also called for a greater effort to build a bigger army, pointing out it can fund 60 Afghan troops for the cost of keeping one foreign soldier in Afghanistan.

Last year, the Taliban carried out around 140 suicide bombings across the country. Many Afghans commonly react by blaming the presence of foreign troops for the attacks and their own government for failing to stop them.

A U.S. military official and counter-insurgency expert said on Wednesday the tide of conflict was running against the United States and its allies in Afghanistan.

In the southwestern Afghan province of Nimroz, Afghan and U.S.-led coalition forces killed 41 Taliban militants in a ground and air assault on Wednesday, the provincial governor said.

"The Taliban were travelling by car when the operation was launched. Forty-one Taliban were killed," he told Reuters.

Nimroz, a sparsely populated desert area bordering Pakistan and Iran, has seen an increase in Taliban activity in recent months as fighters spill over from neighbouring Helmand province where mostly British forces are fighting the insurgents.

A roadside bomb also killed three Afghan police officers and wounded four more in Wardak province, just southwest of Kabul, on Thursday, a provincial intelligence official said. (Reporting by Kabul newsroom and Saeed Ali Achakzai in Spin Boldak; Writing by Jon Hemming; Editing by Jeremy Laurence)

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