KABUL, Oct 6 (Reuters) - A suicide bomber attacked a convoy of foreign troops in the Afghan capital on Saturday, a witness said, but police had no immediate reports of any casualties.

"I can see fire and smoke coming from the foreign vehicles," said the witness, Ghiasuddin Barez.

The head of Kabul criminal investigations said he was on his way to the scene and had no information yet on the number of casualties.

It was the third suicide bombing attack in Kabul in the past eight days and came after a Taliban leader in southern Afghanistan, Mullah Mansour Dadullah, appeared in a video on Wednesday exhorting insurgents to carry out suicide attacks to drive foreign forces from Afghanistan and overthrow the government.

Mullah Mansour took over as commander of Taliban forces in the southern province of Helmand in May after his brother, Mullah Dadullah, was killed in a raid by British special forces.

The feared Mullah Dadullah had been responsible for a wave of suicide bombings and beheadings that shocked Afghans.

After suffering heavy casualties in conventional battles, Taliban rebels have grown increasingly reliant in the last two years on suicide attacks aimed at convincing ordinary Afghans their government and its Western backers cannot provide security.

Afghanistan is going through its worst period of violence since U.S.-led and Afghan forces toppled the Taliban in 2001.

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