The government will not release a prominent militant suspect despite threats against its security forces by the top commander of the Pakistani Taliban, the Interior Ministry chief declared Sunday.

Rehman Malik also laid out what happened during a bloody encounter in Pakistan's northwest between militants and paramilitary forces Saturday, in which 21 people were killed.

Meanwhile, a suicide bomber wounded at least four other people elsewhere in the volatile region — an attack possibly motivated by sectarian rivalry, officials and a witness said.

According to Malik, a Frontier Constabulary convoy heading to examine a post was lured into a trap by five men acting as local guides. The convoy stopped at a roadblock in the Zargari area of Hangu district, and insurgents suddenly appeared.

The five local men said they would go speak to them, but then disappeared.

An ensuing clash killed 16 of the troops, including Karim Khattak, a top local commander for the Frontier Constabulary. Five militants also were killed, including their commander, whom Malik described as an Afghan Taliban.

One of the Frontier Constabulary members hid in a ravine and escaped to tell the tale.

The skirmish was one of the deadliest for Pakistan's security forces in recent months, and it underscored the threat Islamist insurgents pose in parts of the country's northwest.

Malik said tensions have spiked since the government's arrest last week of a man called Rafiuddin, allegedly a top aide to Pakistani Taliban leader Baitullah Mehsud.

According to Malik, Mehsud is furious over Rafiuddin's arrest and has directed his fighters to kidnap security forces in retaliation and ordered police to release his aide.

The Pakistani Taliban have reacted to the arrest, including staging a siege of a police station, and taking, according to Malik, 19 government employees hostage.

On Sunday, Pakistani Taliban spokesman Maulvi Umar demanded the release of Rafiuddin and other captured militant suspects in return for the hostages' freedom. Umar described Rafiuddin and a cleric and Islamic school teacher.

But Malik rejected the militants' demands.

"No terrorist will be released, whatever the price we have to pay," he said.

He ruled out a full-fledged military operation in Hangu, but promised that the government would hunt down the insurgents responsible for the ambush on the convoy.

The Saturday clash occurred in an area wedged between some of Pakistan's tribal regions.

The tribal regions are considered Pakistan's main havens for Taliban- and al-Qaida-linked fighters, but militants have recently increased activities in other parts of the northwest bordering Afghanistan.

Saturday's clash also came as Adm. Mike Mullen, the chairman of the U.S. Joint Chiefs of Staff, visited Pakistan on an unannounced one-day trip.

Mullen met several top political and military leaders and discussed security issues, according to his spokesman. Mullen recently said militants are flowing into neighboring Afghanistan more freely this year compared with last year because Pakistan is not putting enough pressure on them.

Malik said Sunday that the government has credible information that Afghan Taliban were also operating in Pakistan, and said the countries must develop a solid border security system.

"To save Afghanistan is to save Pakistan and to save Pakistan is to save the world," Malik said. "Without securing the western border we will never be able to eliminate this menace of extremism and terrorism."

The suicide bomber Sunday struck in a main road in Dera Ismail Khan, a city south of Hangu district, near a pickup truck carrying Shiite Muslims returning home from a religious conference, said Abdul Nasir, driver of the vehicle who escaped unharmed.

The bomber was hiding on the side of the road when he suddenly came forward and detonated his explosives, Nasir said.

"He chanted 'God is great!' and then there was an explosion," Nasir said.

After the blast, the body of the suspected bomber lay on the road, but most of his explosives belt appeared intact. Police officer Salahuddin Kundi, who confirmed the wounded toll, said experts were called in to defuse the rest of the explosives on the attackers' body.