Armed rebels in Syria say they have shot down a military helicopter over the capital, Damascus. The Free Syrian Army said the aircraft had been firing at people in the north-eastern district of Jobar, and that it had crashed in neighbouring Qabun. State television confirmed that a helicopter had come down in Qabun. On Sunday, opposition activists said government forces had massacred more than 300 people during their assault on the south-western suburb of Darayya. Video footage and photographs have emerged, showing scores of bodies, including those of women and children. State media said the Darayya area was being "cleansed" of armed terrorist groups. The chairman of the UN Human Rights Council's Independent Commission of Inquiry on Syria, Paulo Pinheiro, told the BBC the killings might constitute a war crime. Military offensive According to opposition activists, the military helicopter was shot down as it was bombarding Jobar, as heavy fighting broke out between rebels and government forces. Witnesses told Reuters news agency the helicopter burst into flames after being hit by a projectile during an exchange of fire. Activists posted video footage online which appears to show a burning helicopter crashing to the ground. Rebels can be heard shouting: "God is greatest." State TV confirmed that a helicopter had crashed near the al-Ghufran mosque in Qabun, though it did not say if it had been shot down. The helicopter appears to have been taking part in an increasingly fierce attempt by government forces to regain control of the capital's suburbs. President Bashar al-Assad has called the uprising a "foreign plot against the whole region", and vowed to defeat the conspiracy at "any price".