PESHAWAR, Pakistan — A bomb blast at a mosque in Pakistan's northwestern tribal belt has killed 29 people including some militants, underscoring the relentless security threat here even as Pakistani-US cooperation against extremism appears on the upswing.

Thursday's attack in Khyber tribal region came as US special envoy Richard Holbrooke met with Pakistani leaders in Islamabad, the capital. It also followed revelations that Pakistani authorities have been picking up Afghan Taliban leaders on their soil, and as the US staged its latest missile strike in Pakistan's northwest.

The bomb explosion tore through a mosque in the Aka Khel area of Khyber, killing at least 29 people and wounding some 50 others, local official Jawed Khan said. Earlier reports had said the blast occurred in the Orakzai area at a cattle market.

The two areas border one another, and the market is apparently near the mosque.

Officials were still investigating whether the explosion was caused by a suicide bomber or a planted device.

No group claimed responsibility, but Khan said the dead included militants from Lashkar-e-Islam, an insurgent group in Khyber that has clashed with another militant outfit known as Ansarul Islam. Both espouse Taliban-style ideologies.

Earlier this week, officials confirmed that a joint CIA-Pakistani security operation had captured the No. 2 Afghan Taliban commander, Mullah Abdul Ghani Baradar, in the southern Pakistani city of Karachi.

On Thursday, an Afghan official told The Associated Press that around the same time — some two weeks ago — two Taliban leaders from northern Afghanistan also were arrested in Pakistan by Pakistani authorities. Also, officials said up to nine al-Qaida-linked militants were arrested in a series of raids overnight Wednesday in Karachi.

The US and Pakistan have said very little on the record about the arrests, but they could signal a shift in Pakistani policy. Pakistan has long frustrated the Americans by either denying that the Afghan Taliban use its soil or doing little to root them out.

The arrests could mean that Pakistan has decided to turn on the Afghan Taliban, a group that it helped nurture as a strategic ally against longtime rival India, though some suspect the Pakistanis were forced to act because the U.S. had solid intelligence on Baradar that it could not deny.

During a briefing with reporters in Islamabad, Holbrooke declined to give specifics on Baradar's arrest, and swatted off attempts to link its timing to efforts to negotiate with the Taliban or an ongoing U.S-led offensive in southern Afghanistan's Helmand province. (AP)