The bomber targetted a police vehicle in a bazaar in Deh Rawud district, the provincial police chief told Reuters.

"Seventeen civilians and four policemen died in the attack. Thirty seven more civilians and five police have been wounded," Juma Khan Himat said by phone, adding the death toll could rise.

Himat said most of the civilian victims were children.

He blamed Taliban insurgents for being behind the attack, part of a rising tide of violence in Afghanistan, where the militants largely rely on such tactics as part of their campaign against the foreign troops and the Afghan government.

The Taliban could not be immediately reached for comment.

Over 800 civilians have been killed in Afghanistan so far this year, according to the United Nations and Afghan officials.

Growing insecurity has added to the rising frustration of many Afghans nearly seven years since U.S.-led troops overthrew Taliban's government after it refused to hand over al Qaeda leaders who had masterminded Sept. 11 attacks on America.

The violence comes despite the increasing presence of foreign forces currently numbering more than 70,000, and top Taliban as well as al Qaeda leaders are still at large.

It has prompted some Western politicians to warn recently that the country may slide back into anarchy.

Since 2006 when the Taliban regrouped, Afghan and foreign troops have been locked in almost daily clashes with the militants who have some sanctury in the lawless tribal regions of Pakistan.

In one of the latest such clashes, five Pakistani Taliban were killed and 13 more wounded after they infiltrated a remote district in northeastern Nuristan on Saturday, interior ministry spokesman Zemarai Bashari said.

The fighting was going on Sunday and locals were helping the police for defending the district, he said.