(Adds background) ISLAMABAD, June 5 (Reuters) - A bomb blast killed around 40 worshippers attending Friday prayers at a mosque in a remote area of northwest Pakistan, a senior official told Reuters. "Around 40 people are killed. The death toll is 40. We have no idea as yet how many have been wounded," Atif-ur-Rehman, the senior-most government administrator in Upper Dir, where the explosion occured, said. Upper Dir is close to Swat valley where Pakistani security forge launched a major offensive last month to expel Taliban militants. Pakistani officials have warned that militants would strike back. U.S. special envoy Richard Holbrooke was due to hold talks with army chief General Ashfaq Kayani on Friday. Dir's blast is the ninth bomb attack in the country since the offensive began, and the Taliban has threatened more revenge. (Reporting by Zeeshan Haider, writing by Simon Cameron-Moore; Editing by Sanjeev Miglani)
U.S. special envoy Richard Holbrooke (C) is briefed by a Pakistani official while visiting internally displaced persons at the UNHCR (United Nations High Commission for Refugees) Sheikh Shahzad camp in district ...
