Pakistani police say that a powerful suicide bomb blast hit the funeral of a slain Shi'ite Muslim leader in a northwestern town on Friday, killing at least 28 people and wounding scores of others.

Police and eyewitnesses say hundreds of Shi'ites were attending the funeral of a local leader in the town of Dera Ismail Khan when a suicide bomber hit the procession.

The powerful explosion is said to have killed most of the people instantly. Hospital officials say that a number of people are critically wounded and the death toll is expected rise. No one claimed responsibility but police and leaders of the minority Shi'ite community suspect Sunni Muslims are behind the violence.

Witnesses say gunfire broke out in the city when outraged members of the Shi'ite community fired on police rushing to the scene. The rioting left several people dead.

A top government official, Syed Mohsin Shah, says that the civil administration has called up troops to bring the situation under control.  He says a curfew has been imposed and soldiers have started patrolling the streets to discourage rioting.

Sectarian tensions have been running high in Deral Ismail Khan for many years. But, in recent months, Sunni militants have introduced a new pattern of attacks on rival Shi'ite groups. They first kill a local Shi'ite leader and then a suicide bomber hits the slain man's funeral procession.

Critics say that rising sectarian attacks could further weaken the security situation in northwestern Pakistan where the government is already under intense international pressure to crack down on pro-Taliban militants.

In recent days, the northwestern scenic valley of Swat has been the focus of international concern because of a peace deal signed by Pakistani authorities and local Taliban militants. Under the agreement, the government will facilitate installation of an Islamic system of justice if militants lay down their arms.

Pakistan's Foreign Ministry spokesman Abdul Basit has rejected international criticism of the peace deal, saying the understanding is part of the government's efforts to eliminate extremism.

"Establishing peace, security and stability are matters of highest priority for the [Pakistani] government. Pakistan will use all necessary means to achieve these objectives," he said.  "Therefore, speculation in the matter [peace deal] will not be helpful."

U.S special envoy for Afghanistan and Pakistan Richard Holbrooke has said that militants in Swat pose a direct threat to Pakistan and the United States. In a television interview on Thursday, the senior American diplomat said that as he puts it, Washington is worried the agreement with militants in Swat will turn into surrender.    

Holbrooke says that he raised these concerns during a phone call with Pakistani President Asif Ali Zardari. He says that the Pakistani leader has assured him the deal was an "interim arrangement" to stabilize the region.

Most of the area in the northwestern district of Swat is said to be under the control of the Pakistani Taliban who have long demanded introduction of their strict version of Islamic law. However, the fate of the peace deal depends on the outcome of talks between militant leaders and representatives of an outlawed Islamic group that signed the agreement with the provincial government on behalf of the Taliban.