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 on the sidelines of the UN General Assembly meeting in New York, but he added that the removal of his ally Assad would turn Syria into an extremist safe haven.Discussions of political reform in Syria should come after the threat of extremism in Syria has been removed.Separately, Iran's state news agency IRNA said Rouhani will cut short his visit to New York and return to Tehran for the funeral of Iranians killed in the haj tragedy in Saudi Arabia, Iran's state news agency IRNA reported on Sunday."Some of his meetings and scheduled programs will be canceled and he is returning (to Tehran) on Monday afternoon," IRNA quoted a senior official from Rouhani's office as saying.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" and shares the same desire to defeat the threat of Islamic State."If the priority is not to defeat terrorism, we are making a big mistake," Rouhani said.He added that Russia's and Iran's positions on Syria were "almost identical."Failing to address the problem of Islamic State properly and forcing out the government would result in the group taking control of Damascus, Rouhani said.