The US and partner nations are carrying out the first air strikes against Islamic State targets in Syria, the Pentagon said on Monday, in ongoing operations that mark the opening of a new, far more complicated front in a battle against the militants.

"I can confirm that US military and partner nation forces are undertaking military action against (Islamic State) terrorists in Syria using a mix of fighter, bomber and Tomahawk Land Attack Missiles," Rear Admiral John Kirby, Pentagon press secretary, said in a statement.

"Given that these operations are ongoing, we are not in a position to provide additional details at this time."

Kirby did not disclose which nations joined the United States, which has been building a coalition to combat an extremist Sunni Muslim force that has seized large expanses of territory in Iraq and Syria and proclaimed a caliphate erasing borders in the heart of the Middle East.

The strikes took place hours before Obama goes to New York for the U.N. General Assembly where he will try to rally more nations behind his drive to aggressively take on Islamic State.

"We will provide Obama had shied away from getting involved in Syria's civil war a year ago, seeing no positive outcome for the United States, but the rise of Islamic State and the beheading of two American captives forced him to change course.

General Lloyd Austin, commander of the US military's Central Command, made the decision to conduct the strikes under authorization granted to him by Obama, Kirby said.

"We will provide more details later as operationally appropriate," Kirby said.

Ongoing strikes by the US and partner nations in Syria could include around 20 targets, a US official said on Monday, speaking on condition of anonymity.

The official said some of the strikes employed Tomahawk missiles launched from at least one US ship at sea.

Saudi, UAE, Jordan and Bahrain 'involved' in strikes

Saudi Arabia, the United Arab Emirates, Jordan and Bahrain were involved in US air strikes launched in Syria, the official said, although their exact roles in the military action were unclear.

Qatar played a supporting role in the air strikes, the official added. – Reuters