Read Christina Lamb's weblog from Kabul

President Karzai of Afghanistan has demanded a rethink of the War on Terror, saying that his country cannot withstand the recent surge in violence, which has killed around 600 people since the beginning of May.

Speaking after the death of four American soldiers and the appearance of a video message from Ayman al-Zawahri, Osama bin Laden's deputy, calling for Afghans to make a stand against foreign forces, a frustrated Mr Karzai acknowledged the weakness of his Government.

More than 1,000 people, including at least 40 foreign soldiers, have been killed in Afghanistan since the turn of the year during the worst period of sustained unrest since the fall of the Taleban in November 2001. The violence is expected to worsen this summer as Nato prepares to take over the military mission to the country.

"We know the causes," said Mr Karzai of the deteriorating security situation. "There are shortcomings and inabilities in our system, that weakness is present all over the country. But there is no doubt it is largely because of foreign factors, terrorism and planned and co-ordinated attacks."

Mr Karzai did not say what he meant by "foreign factors" but Afghan authorities frequently complain that their Pakistani counterparts are not doing enough to secure the porous, mountainous border to prevent Taleban and al-Qaeda fighters moving in and out of the country.

The four US soldiers killed yesterday died trying to stop "enemy movement" in the eastern province of Nuristan, whose mountains, forests and gorges run unpatrolled for hundreds of miles along the edge of Pakistan's Chitral province.

Asked whether he thought the US-led fight against the Taleban in Afghanistan was failing, Mr Karzai recommended a "thorough look" at the strategy of the War on Terror.

"I strongly believe... that we must engage strategically in disarming terrorism by stopping their sources of supply of money, training, equipment and motivation," he said. "This ’War on Terror’ has been limited to Afghanistan soil. We can’t tolerate it forever... in the past three weeks five, six hundred people have died in the country. We want an end to this, a basic end to this."

US military commanders said earlier that the four soldiers died and a fifth was wounded fighting in the Kamdesh district of Nuristan, a nearly roadless province of mountains and steep, terraced valleys that for centuries has been fiercely independent of the rest of Afghanistan.

"Coalition forces attacked enemy extremists in a remote area of the Kamdesh District while conducting security operations to interdict enemy movement through northern Nuristan," the U.S. military said. "During the mission, four US soldiers were killed."

Airstrikes were called in at the end of the raid, but there was no confirmed report of any militant casualties. The battle formed part of Operation Mountain Lion, a joint US-Afghan campaign launched in eastern Afghanistan to hunt al-Qaeda and Taleban forces in the area.

The al-Zawahri video, which carried the stamp of al-Qaeda's in-house media unit, Al-Sahab Productions, appeared to have been made on May 30, the day after an American lorry ran out of control in Kabul, killing five people and prompting anti-Western riots that cost the lives of a further 20 people in Kabul.

Entitled "American Crimes in Kabul", the tape shows the Egyptian cleric seated in front a black backdrop with an automatic rifle. Unlike other tapes, it carried no English subtitles, only translations into Farsi and Pashtun, suggesting it was aimed at Afghans rather than the West.

"I am calling upon the Muslims in Kabul in particular and in all Afghanistan in general and for the sake of God to stand up in an honest stand in the face of the infidel forces that are invading Muslim lands," said al-Zawahri.

"I direct my speech today to my Muslim brothers in Kabul who lived the bitter events yesterday and saw by their own eyes a new proof of the criminal acts of the American forces against the Afghan people."

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