NEW YORK - Iran's President Hassan Rouhani said on Sunday fighting radical militants like Islamic State in Syria is the top priority and if they are to be defeated then President Bashar al-Assad's government 'can't be weakened.' 'This does not mean that the Syrian government does not need reform ... Of course it does,' Rouhani told an audience of US think-tanks and journalists, but he added that the removal of his ally Assad would turn Syria into an extremist safe haven. Referring to air strikes by a US-led coalition on Islamic State targets in Syria, Rouhani said defeating the militants was 'not feasible through air operations only.' He said Russia 'is ready to fight terrorism.'