US President Barack Obama has vowed to bring to justice those who killed the US ambassador to Libya during protests against a film that mocks Islam. But he told reporters that the attack on the American consulate in Benghazi would not break the bonds between the US and the new Libyan government. His election rival Mitt Romney criticised his handling of the crisis. Ambassador J Christopher Stevens reportedly died of smoke inhalation after a crowd stormed the consulate. Three other Americans were also killed and the consulate set ablaze. Rocket-propelled grenades were reportedly fired during the assault on Tuesday night. Charred vehicles could be seen parked near the damaged buildings on Wednesday. A US marine anti-terrorism team is being sent to Libya to bolster security after the attack, a US defence source told reporters in Washington. Protesters against the film attacked the US embassy in Cairo on Tuesday night.