A homicide bomber attacked the funeral of a slain Shiite Muslim leader in northwestern Pakistan on Friday, killing 28 people and triggering deadly rioting, officials said.

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Rising sectarian attacks threaten to further destabilize nuclear-armed Pakistan just as it faces intense international pressure to crack down on Islamist militants.

Meanwhile, a top U.S. official said Washington was worried that a five-day-old cease-fire in the restive Swat valley could "turn into a surrender" to Taliban insurgents behind beheadings, the bombings of schools for girls and attacks on security forces.

Friday's explosion struck a 1,000-strong crowd streaming toward a graveyard in Dera Ismail Khan for the burial of Sher Zeman, a Shiite leader who was gunned down in the city the day before.

Police official Ishtiaq Marwat said a homicide attacker killed at least 28 people and wounded more than 60 others, leaving shoes and torn clothing littering a bloodstained street. Some of the dead and injured were taken to the hospital in wooden handcarts.

Gunfire broke out afterward and police said angry Shiites fired on officers rushing to the scene. Marwat said two Sunni Muslim residents had been shot dead in the rioting.

There was no immediate claim of responsibility, but past attacks on Pakistan's minority Shiites have been carried out by extremists from the majority Sunni community who regard the sect as heretical.

Fayyaz Hussain, a local Shiite leader, said extremists were trying to start a wider sectarian conflict.

"This attack is yet another attempt to force us to leave Dera Ismail Khan, but we will face the situation and will stay here," Hussain said.

Relations between the two communities are under growing strain following a series of attacks. A car bomb killed 29 people and wounded scores near a Shiite mosque in Peshawar in December. On Feb. 5, a suicide bomber killed 24 at a Shiite mosque in a central city.

Much of the bloodshed has been in the northwest, where the Taliban and other hardline Sunni groups have seized control of swaths of territory, despite a series of military offensives.

International concern is focused on Swat, where troops and militants have been observing a cease-fire since authorities announced a deal to introduce Islamic law if militants lay down arms.

Richard Holbrooke, the new U.S envoy for Afghanistan and Pakistan, said Thursday that he had raised concern about the deal during a phone call with Pakistan's president.

Holbrooke told CNN that President Barack Obama was worried "that this deal, which is portrayed in the press as a truce ... does not turn into a surrender."

He said Zardari told him during Thursday's phone call that the pact was an "interim arrangement" while Pakistan stabilizes the situation.

"He doesn't disagree that the people who are running Swat now are murderous thugs and militants and they pose a danger not only to Pakistan, but to the United States and India," Holbrooke said.