PESHAWAR – A bomb blast at a mosque in Pakistan's northwestern tribal belt killed 29 people including some insurgents on Thursday, underscoring the relentless security threat here even as Pakistani-US co-operation against extremism appears on the upswing. The attack in Khyber tribal region came as US special envoy Richard Holbrooke met with Pakistan's Prime Minister Yousuf Raza Gilani in Islamabad, the capital. It also followed revelations that Pakistani authorities have been picking up Afghan Taliban leaders on their soil, a longtime US demand. The 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 are 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 insurgents from Lashkar-e-Islam, an insurgent group in Khyber that has clashed with another insurgents 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. Pakistan has raised fresh concerns to the US about Afghan refugees and fighters fleeing across the border to escape a major US-led offensive in neighbouring Afghanistan. Gilani raised the matter in talks with Holbrooke, during talks in Islamabad, his office said. About 15,000 Afghan, US and NATO troops are conducting Operation Mushtarak (Together) in southern Afghanistan against about 400 to 1,000 Taliban fighters. The operation has been described as the biggest assault since the 2001 US-led invasion. The offensive is targeting a major drug-producing area in Helmand province, which borders Pakistan's insurgency-rife Baluchistan province. Northwest Pakistan is a stronghold of Taliban and al-Qaeda-linked fighters, hundreds of whom fled the 2001 US-led invasion of Afghanistan. Gilani expressed hope that 'Pakistan's concerns on account of spillover of refugees and militants from Helmand into southwestern Baluchistan and the northwest will be kept in view by the US and ISAF forces,' his office added. He called for 'enhanced coordination and cooperation' with Pakistan's armed forces to deal with the situation.