Pakistani fighter jets bombed suspected Taliban hideouts on Monday and a stray mortar crashed into a mosque during prayers as violence in the volatile northwest claimed more than 20 lives.

The air strikes hit a guest house being used by militants in the village of Kani Guram, in South Waziristan, where government forces are readying an offensive against Pakistani Taliban leader Baitullah Mehsud. Four militants were killed, three intelligence officials said.

The officials spoke on condition of anonymity because they were not authorized to release the information.

It was not possible to independently confirm the casualty counts or the identities of those reported killed as journalists have little access to the remote, dangerous region.

Late Sunday, a stray mortar shell hit a mosque during prayers in Azam Warsak in South Waziristan, killing three tribesmen and wounding seven, intelligence officials and a witness said.

"The mosque was destroyed and we could hardly bring out the dead and injured," said a man who gave his name as Wazir. He escaped the shelling unharmed.

It was not immediately clear who fired the mortar, but intelligence officials said it appeared to be aimed by at a nearby military outpost, presumably by militants.

On Monday, security forces launched an early morning raid on a suspected militant hideout in Tank, a small city near South Waziristan, killing two suspected militants and arresting nine others, senior police officer Abdul Rasheed said.

Meanwhile, 15 militants died in overnight clashes with a local tribal militia in the tribal region of Kurram, north of Waziristan, a tribal elder and a lawmaker in the region said.

Ali Akbar Toori and lawmaker Sajid Toori said two tribal militiamen were killed and 35 were injured in the attack, which appeared to be an attempt by militants to take over the area.

Pakistan says it is preparing for a major offensive aimed at eliminating Mehsud and his militant network in South Waziristan, which lies in the mountainous tribal belt along Pakistan's long border with Afghanistan where the government wields little control.

Mehsud's group has been blamed for a string of deadly suicide bombings across the country that have killed more than 100 people in the past month. On Sunday, a militant ambush claimed by Mehsud killed a dozen soldiers in North Waziristan.

On Sunday, the government announced a 50 million rupee ($615,000) reward for information leading to Mehsud's capture or death. Smaller amounts were offered for information on his top lieutenants.

The government's military campaign is strongly supported by Washington. It is seen as a test of Pakistan's determination to confront an insurgency that has grown after earlier military operations failed to finish the job and peace deals with the Taliban collapsed.

It comes as the military pursues a two-month-old offensive to oust Taliban militants from the Swat Valley region, also in the northwest.