Six Pakistani nationals have been charged with planning an act of terrorism in Britain, as part of a plot that included training in Pakistan, preparing for suicide bombings and constructing an explosive device, the police said. The men, aged 25 to 32, are all from Birmingham, Britains second-largest city. Four were charged with preparing for an act of terrorism in the U.K., and the other two were charged with failing to disclose information. A seventh man was being questioned, the West Midlands police force said in a statement. The men were arrested last week in a raid led by the counterterrorism unit of Britains West Midlands police. The police have given no details of the alleged plot, but a police statement said that the arrests were part of a major operation prompted by intelligence work, a formulation that has often been used to describe a lengthy period of surveillance, including phone-tapping, by police undercover units and Britains secret intelligence and security agencies, MI5 and MI6.