A Taliban suicide bomber rammed a car into a convoy of NATO forces close to the airport in the Afghan capital, during a visit by US Defense Secretary Robert Gates to assess the rising violence.

No casualties were reported among NATO troops in the morning rush hour blast on a road just outside Kabul's international airport, an alliance spokesman said. NATO said the blast wounded 22 Afghan civilians.

A spokesman for the Taliban said the militant Islamic group carried out the attack to "welcome" Gates, who arrived in Kabul yesterday evening.

The spokesman for NATO, which runs a force separate to the US military in Afghanistan, said that one of its vehicles was damaged in the blast.

Today's blast reflected a rising level of violence in the past two years in Afghanistan, the bloodiest period since the Taliban's removal from power in 2001, that has left an increasing number of Afghans frustrated with the lack of progress.

Afghanistan's army chief, General Bismillah Khan, asked Gates for more security trainers and equipment to fight the Taliban insurgency.

"The biggest problem is we don't have enough mentors, enough advisers," Khan told Gates after the two toured the Kabul Military Training Centre. "I need your prompt attention on this matter."

"The US government has been more than generous but our weapons are not adequate," he told the Pentagon chief through a translator.

Khan specifically asked for small arms, mortars and armoured vehicles.

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