Three positions of the Nusra Front between Idlib and the northwestern province of Aleppo were hit by the US-led airstrikes, said the Observatory, adding that the Islamic State (IS) militants had evacuated their positions in the city of Manbej in the countryside of Aleppo ahead of the strikes.

In the city of al-Bab in Aleppo, the IS fighters imposed a curfew on Monday evening that lasted till Tuesday morning, according to the Observatory.   

US and its partners began striking IS targets in Syria with a mix of fighters, bombers and Tomahawk missiles before daybreak on Tuesday.

Saudi Arabia, Jordan and the United Arab Emirates have reportedly participated in the air strikes.

The Syrian government said it was informed about the strike ahead of its occurrence.

Earlier on Tuesday, US Defense Department announced that US and allied nations launched air attacks against the IS Sunni extremist group in Syria.

"Forces of United States and allied nations have begun attacks on IS in Syria using a combination of fighters, bombers and Tomahawk missiles," a news agency quoted Pentagon spokesperson John Kirby as saying on Twitter.

US Central Command, in charge of operations in the Middle East, took the decision to begin the attacks after receiving President Barak Obama's authorisation, Kirby said in a brief statement without providing further details "given that the operation is in progress", the report said.

Two weeks ago Obama said the US would extend into Syria its air operations against the IS in Iraq.

The IS offensive against religious and ethnic minorities in northern Iraq over the past several months, coupled with grisly videos of the jihadis beheading captured Westerners, galvanised US into attacking the group.

Obama has insisted this operation would be different from the recent wars in Iraq and Afghanistan because it would not involve the deployment of US troops on the ground.

But some senior military officials have said it was impossible to defeat the IS without ground combat.

US launched its first attack against the IS near Baghdad just over a week ago.

Until then the strikes had been limited to the jihadis' positions in the north for the protection of US diplomats and other officials in the area and for humanitarian reasons.

At the request of the President, US Congress has also authorised the arming of Syrian rebels fighting the IS, a "key element" of Obama's strategy in stopping the jihadis.

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