The Pentagon says the U.S. and partner nations have begun airstrikes in Syria against Islamic State militants, using a mix of fighter jets, bombers and Tomahawk missiles fired from ships in the region.

Pentagon Press Secretary Rear Adm. John Kirby says that because the military operation is ongoing, no details can be provided yet. He says the decision to strike was made early Monday by the military.

Among the nations joining in the strike in Syria are Arab nations Jordan, Saudi Arabia, Bahrain and United Arab Emirates.

The strikes are part of the expanded military campaign that President Barack Obama's authorized nearly two weeks ago in order to disrupt and destroy the Islamic State militants, who have slaughtered thousands of people, beheaded Westerners, including two American journalists, and captured a large swath of territory stretching from within Syria to land across northern and Western Iraq.

Congress cleared the way last week for the president to approve attacks on Syria against Islamic State. The approval also includes equipping and training Syrian rebels. The president has said that no combat forces would be on the ground in Syria fighting the terrorist group.

The head of Syria's main Western-backed opposition group on Thursday called for immediate U.S. airstrikes targeting the Islamic State group in Syria, saying urgent action was required "to avert catastrophe."

Hadi Bahra spoke at a press conference at the United Nations as intense fighting continued between the extremist group and Kurdish forces near the Syrian frontier with Turkey, triggering a massive surge of refugees fleeing across the border. The Islamic State group offensive in the area has already sent 130,000 refugees to seek safety in Turkey in the last few days.