A suicide bomber tonight blew himself up at the office of a police emergency response unit in the heart of Pakistan's capital,killing two policeman and injuring four others. The suicide bomber was challenged when he tried to enter the compound of the Rescue 15 unit of Islamabad Police at 8.30 PM. The attacker blew himself up when a guard opened fire at him. Additional Deputy Commissioner General Rana Akbar Hayat told reporters that two policemen were killed and four others injured in the blast. One policeman died on the spot while another succumbed to his injuries on the way to hospital. Witnesses said they had seen limbs of the bomber scattered across the compound. Officials said the attacker tried to enter the compound from a nearby residential area. They said casualties could have been higher if the attacker had managed to enter the building,where about 15 policemen were on duty. The anti-terrorism squad and police commandos cordoned off the area to collect evidence. Islamabad Police had recently increased security measures around police stations and emergency response units to thwart attacks. Interior Minister Rehman Malik condemned the blast and ordered a probe into the incident. The attack came a day after 40 people were killed in a suicide bombing at a crowded mosque in the Dir area of the troubled North West Frontier Province. Pakistan has witnessed a series of bombings and suicide attacks since the army launched an operation against the Taliban in Buner,Dir and Swat districts of the NWFP. Militants have carried out suicide attacks in Peshawar,Dera Ismail Khan and Lahore in the past few weeks.