A bomb blast at a bus station in northwestern Pakistan has killed at least four people and injured sixteen others, police say.

The remote-controlled bomb exploded at a busy bus stop in Tirah Market in Kohat, a garrison town in North West Frontier Province (NWFP). Rescue workers shifted the injured to nearby hospitals.

The death toll is expected to rise as some of the wounded are reported to be in critical condition.

Police officials say the attack could be in retaliation for the army's operations in the Swat Valley region.

Security analysts had earlier warned that militants -who have suffered heavy losses-- would take the war into Pakistani cities to maximize civilian carnage to pressure the government to halt the crackdown on militants in the northwest.

Several Pakistani cities including capital Islamabad and Peshawar have been rocked by bomb blasts and dozens have been killed since the army initiated its offensive in the Swat region in late April.

Last week at least 40 people were killed and more than 250 others injured in a massive bomb attack targeting the police offices in Pakistan's eastern city of Lahore.

Taliban spokesmen have claimed the responsibility for the attacks inside Pakistan's main cities saying they were revenge for the army's assault.

Security forces claim that the month-long anti-militant campaign in Swat could be over in few days.

HE/DT