A bombing outside the emergency gate of a hospital crowded with Shiite Muslim mourners in Pakistan's volatile northwest killed at least 23 people and wounded 15 Tuesday, officials said.

The motive for the blast in Dera Ismail Khan District appeared to be sectarian, with the Shiites the apparent targets, said Mohsin Shah, a top district official.

The attack occurred a day after Pervez Musharraf stepped down as Pakistan's president, adding to uncertainty about how the country's new government will deal with growing extremist violence.

It also came as more killings were reported in a northwest tribal region where military clashes with insurgents have reportedly left hundreds dead and spurred promises of militant revenge.

There was no immediate claim of responsibility for the explosion at Dera Ismail Khan District Hospital, but it came amid ongoing tensions between Sunni and Shiite Muslims in the area that have spawned several targeted killings in recent weeks, according to Shah.

He said a Shiite man shot earlier Tuesday in the center of the city was taken to the hospital where he died from the wound. "Dozens of people from the Shiite community had gathered at the hospital where the bomb went off," Shah said.

Naveed Malik, the provincial police chief in North West Frontier Province, told the Geo news channel that the attack may have been launched by a homicide bomber.

He said 23 people died and between 15 and 20 more were wounded.

"As you know, there has been sectarian tension here, and this wave keeps decreasing and increasing, so now we are having this sectarian wave a bit more and this is the result," Malik said.

Pakistan's northwest has been plagued for years with militant and sectarian violence.

The country is majority Sunni, but has a sizable Shiite population. Most people from both sects live together peacefully, though extremists from both sides target each others' activists and leaders. The Sunni-Shiite schism over the true heir to Islam's Prophet Muhammad dates back to the seventh century.

Pakistan's northwest tribal regions along the Afghan border, meanwhile, are considered havens for Taliban and Al Qaeda-linked insurgents, many of whom are believed involved in attacks on U.S. and NATO forces in Afghanistan.

In recent weeks, the military has battled insurgents in Bajur tribal region.

On Tuesday, police said security forces backed by helicopter gunships and artillery pounded targeted insurgents in the area, killing 11 suspected militants and five civilians over a 24-hour period.

Security forces stepped up the shelling after militants attacked a paramilitary post at Mamad Gatt near the Afghan border, said Fazal Rabbi, a police commander in Bajur. He said he did not know if any of the paramilitary troops were killed.

Separately, government official Jamil Khan said 13 militants and five paramilitary troops died Tuesday in a clash at a fort in the Nawagai area of Bajur.

Officials say the Bajur operation has killed nearly 500 people and displaced more than 200,000. There has been no independent confirmation of the casualties in the remote area, where the government has limited control and few journalists venture because of its security.

A week ago, a bombing in Peshawar destroyed an air force truck and killed up to 14 people. Pakistani Taliban claimed they staged it in response to recent military offensives.