Police began investigations Friday to determine who ambushed a vehicle carrying government troops in southwestern Pakistan, killing seven soldiers and two police who were escorting them.

The troops were attacked just before midnight Thursday in Quetta, hours after a top U.S. official visited the city, considered a hide-out for the Taliban, police chief Rehmatullah Niazi said. Five soldiers were also wounded in the attack.

"It was an act of terrorism, but we don't know who was responsible," Niazi told The Associated Press. "We are clueless at this stage, but the attackers will soon be traced and arrested as we have opened investigations."

Niazi provided no further details. Abdul Razaq Bugti, a spokesman for the provincial government, said police detained two dozen men in connection with the shooting, although he said none was apparently involved in the attack.

"These people are from nationalist groups, and police rounded them up Friday," he said. "There is a strong possibility that associates from some nationalist group were involved."

Baluchistan, of which Quetta is the capital, has experienced scores of attacks on military and government targets, most blamed on ethnic Baluch tribesmen and nationalist groups who are demanding that the central government grant more royalties and control over resources, such as natural gas, extracted from the province.

It has also seen serious sectarian violence and is used by Taliban militants to launch attacks across the border on Afghan and foreign troops.

The shooting came hours after U.S. Assistant Secretary of State Richard Boucher visited the city and met with provincial Chief Minister Jam Mohammed Yousaf.

Pakistan, a key ally of the United States in its war on terror, is under increasing pressure from Washington and Kabul to help contain the growing violence in Afghanistan, where U.S. forces have been operating since late 2001.

Yousaf told Boucher that Al Qaeda chief Usama bin Laden and Taliban chief Mullah Omar were not hiding in the region, a government statement said.

President Gen. Pervez Musharraf has angrily rejected claims by Afghan President Hamid Karzai that Omar was living in Quetta, insisting that the Taliban leader was in Afghanistan's neighboring Kandahar province, directing the insurgency.

However, Pakistani authorities captured former Taliban Defense Minister Mullah Obaidullah Akhund in Quetta in March.

U.S. Deputy Secretary of State John Negroponte joined Boucher in Pakistan on Friday for talks with Pakistani leaders expected to touch on a judicial crisis clouding Musharraf's plans to extend his eight-year rule as well as Pakistan's role in combating the Taliban and Al Qaeda.