(Updates with NATO comment)

KHOST, Afghanistan, April 25 (Reuters) - A roadside bomb killed seven Afghan soldiers near the border with Pakistan on Wednesday, the latest in a spate of attacks on Afghan security forces.

The soldiers were travelling on a dirt road in the Waza Khaw district of southeastern Paktika province when the bomb blew up their vehicle, provincial governor Akram Khpelwak told reporters.

There was no immediate claim of responsibility but the Taliban, who have vowed to step up their war against the government and foreign troops supporting it, have claimed numerous similar attacks.

Wednesday's attack was the deadliest on Afghan soldiers in many months. About 20 policemen and other security officials have been killed in blasts over the past two weeks.

Four policemen were killed late on Tuesday in an ambush in the generally peaceful western province of Herat. Police blamed the Taliban.

Violence has been increasing this month after a traditional winter lull. Last year saw the worst fighting since U.S.-led forces ousted the Taliban in 2001 and many security analysts warn this year could be worse.

Also on Tuesday, Taliban ambushed and killed four Afghan guards of a foreign construction company in Ghazni province, southwest of Kabul, provincial officials said.

Three workers were abducted and five Taliban killed in a gun battle that followed the attack, police said.

The Taliban claimed responsibility. A spokesman, Qari Mohammad Yousuf, said the Taliban had captured two workers.

Taliban commanders have said thousands of suicide bombers are waiting for orders to strike across the country.

The government and foreign forces say the Taliban are increasingly resorting to suicide and remote-controlled bomb attacks out of desperation.

A senior Afghan security official said on Tuesday Afghan and NATO troops had surrounded more than 200 Taliban in the southern province of Uruzgan and their feared military commander Mullah Dadullah might be among them.

But NATO forces were not involved in the operation, a NATO spokeswoman said on Wednesday, and an Afghan politician from the region said he doubted that Dadullah had been surrounded.

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