At least four people were killed and seven injured when a suicide bomber blew himself up inside a stadium in northwest Pakistan on Sunday, officials said. The bomber was believed to have been targeting a group of refugees from the country's troubled tribal belt who had gone to a government-run camp in the stadium to fill out paperwork ahead of their journey home. "At least four people including one tribal policeman were killed and seven injured when a suicide bomber blew himself up in the limits of Arbab Niaz stadium in Parda Bagh area of Peshawar," senior police official Faisal Mukhtar told AFP. Najeeb U Rehman, another senior police official, confirmed the incident, which took place near a small mosque within the stadium. "It was a suicide blast, around 10 kilograms (22 pounds) of explosives were used," he told AFP. He said the suicide bomber was trying to target the refugees who had fled fighting between the military and the Taliban in the Khyber tribal district. "But he could not reach them because the security forces started firing at him and he blew himself up," Rehman said. Shahid Afridi, a firefighter present at the site during the blast, said a man in his twenties entered the stadium and started firing with a pistol. "The security forces retaliated and then there was a explosion and I fell to the ground," said Afridi, who sustained leg injuries following the blast. Muzafaruddin Sadiq, head of the government-run Lady Reading Hospital, said two of the injured were in critical condition. Thousands of people from Tirah valley in Khyber tribal district fled their homes after the military began an operation against al-Qaida-affiliated militant groups in 2011. Many have sought shelter in government-run camps while others stayed with extended family or in rented accommodation. With the fighting in that district calming down, the government has started a repatriation process that requires them to register themselves so they are cleared to go home. The attack comes two weeks after the Tehreek-e-Taliban Pakistan (TTP) formally ended a ceasefire called to help peace talks with the government. Prime Minister Nawaz Sharif's government began negotiations with the TTP through intermediaries in February in the hope of ending their bloody seven-year insurgency. Since the TTP rose up against the Pakistani state in 2007, more than 6,800 people have been killed in bomb and gun attacks around Pakistan, according to an AFP tally.