A bomb attack has killed three people, including two children, at a Sufi shrine in Pakistan's city of Peshawar.

A police official said the five-year-old boy and a four-year-old girl had come to visit the shrine with their parents.

"Three people have died and 21 are wounded," Asif Iqbal said.

"The bomb was planted in a donkey cart parked near the shrine." The donkey, whose front legs were tied with a rope, was blown to pieces, Iqbal said.

The explosives were planted near the Panj Peer shrine in a densely populated part of the northwestern city, which runs into Pakistan's tribal belt, strongholds of Taliban and al-Qaeda.

"It appears that the target was the worshippers who were gathered here", police official Tahir Ayub said. The shrine houses the graves of several Sufi saints.

Pakistan says 35,000 people have been killed as a result of what it terms terrorist activities in the country since the start of the US-led war on al-Qaeda after the 9/11 attacks on the US.

There was no immediate claim of responsibility for the blast.

Taliban fighters have often targeted such shrines in the past because they consider them a form of idol worship that is forbidden in their strict interpretation of Islam.