Iran's President Hassan Rouhani
Iran's President Hassan RouhaniReuters

Iran's president Hassan Rouhani has criticised the US-led airstrikes in Syria calling them illegal because not previously approved by the Syrian government.

At a New York press meeting of the UN general assembly gathering of world leaders, Rouhani, who is due to hold talks with British PM David Cameron, noted that the Islamic Republic condemns Isis for torturing and killing innocent civilians and stands ready to help fight terrorism.

But he added that US policy is confused because it seeks to undermine the government of Syria's president Bashar al-Assad, a longstanding ally of Shiite Iran, while launching airstrikes on Sunni Islamist militants opposed to the Syrian regime such as Isis and the al-Nusra front.

"This is clearly nebulous and ambiguous at best," he said. "This is a very confusing behaviour and policy."

Raids on Islamic State militants and other al-Qaida-linked groups in northern and eastern Syria started on Monday. Arab states such as Saudi Arabia, Qatar, Bahrain, Jordan and the UAE took part to the airstrikes, which killed at least 50 al-Nusra fighters and 70 Isis militants and injured another 300, according to figures by the Syrian Observatory for Human Rights.

Syria's foreign ministry said the US informed Syria's envoy to the UN that airstrikes would be launched in the war-torn country.

Iran ruled out an alliance with the US to strike Isis, according to Iran's supreme leader Ayatollah Ali Khamenei, although both see the jihadists as a threat.