(Update Sept 23, 11:38) An ISIL fighter said on Tuesday the group will respond to US-led air strikes inside Syria and blamed Saudi Arabia for allowing them to happen.
"These attacks will be answered. The sons of Saloul are the ones who are to be blamed. It happened because of them," he told Reuters, using a derogatory term for Saudi Arabia's royal Saudi family.
In a statement on Tuesday, the US military said it had taken action to disrupt "imminent attack" against the US and Western interests by "seasoned Al Qaeda veterans" who had established a safe haven in Syria. "These strikes were undertaken only by US assets," it said.
The statement said the US had launched strikes from warships in international waters in the Red Sea and the North Arabian Gulf.
It said Bahrain, Jordan, Qatar, Saudi and the UAE had "also participated in or supported the air strikes against (ISIL) targets. All aircraft safely exited the strike areas," it said.
"I can confirm that US military and partner nation forces are undertaking military action against (ISIL) 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."
Another official said at least one US ship had launched surface-to-surface Tomahawk cruise missiles. Armed US drones were also used in the attacks.
The targets included Raqqa city in eastern Syria, the headquarters of ISIL, 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.
A group monitoring the war in Syria said at least 20 ISIL fighters were killed.
Syrian state television said the United States informed Syria's UN representative on Monday that ISIL targets would be hit in Raqqa, which is 400 km (250 miles) northeast of Damascus.
The United States has previously stressed it would not coordinate with the government of President Bashar Al Assad in any way in its fight against ISIL. US President Barack Obama's position has long been that he would like to see Assad leave power, particularly after using chemical weapons against his own people last year.
US officials and the Syrian Observatory for Human Rights, a monitoring group that tracks violence in the Syrian war, said buildings used by the militants, their weapons supplies and checkpoints were targeted in the attacks on Raqqa. Areas along the Iraq-Syria border were also hit.
Rami Abdulrahman, head of the Observatory, which gathers information from a network of activists on the ground, told Reuters in Beirut by phone that at least 50 air strikes had been carried out, with the al Qaeda-linked Nusra Front in northwestern Syria also hit.
Residents in Raqqa had said last week that ISIL was moving underground after Obama signalled on Sept. 11 that air attacks on its forces could be expanded from Iraq to Syria.
The group had evacuated buildings it was using as offices, redeployed its heavy weaponry, and moved fighters' families out of the city, the residents said.
"They are trying to keep on the move," said one Raqqa resident, communicating via the Internet and speaking on condition of anonymity because of safety fears. "They only meet in very limited gatherings."
The addition of Arab allies in the attacks was seen as crucial for the credibility of the American-led campaign. US allies in the Middle East are sceptical of how far Washington will commit to a conflict in which nearly every country in the region has a stake, set against the backdrop of Islam's 1,300-year-old rift between Sunnis and Shi'ites.
As part of US efforts to build the coalition, Secretary of State John Kerry travelled to New York at the weekend, ahead of the start of United Nations General Assembly meetings, for talks with counterparts from Arab and European allies to discuss plans to defeat ISIL and hear their views on how they might participate.
On Monday, Kerry met Saudi Foreign Minister Saud Al Faisal and British Foreign Secretary Philip Hammond, and participated in a meeting of more than a dozen countries, including Arab Gulf States, on the conflict in Libya.
A senior administration official said US plans "to expand our efforts to defeat (ISIL) were discussed without specifics" during meetings but declined to elaborate.
Several Arab states have powerful air forces, including Saudi Arabia and the United Arab Emirates. Saudi Arabia has also already agreed to host US training of Syrian opposition fighters.
But many Gulf Arab states have been reluctant to be seen aggressively joining the US campaign, fearing in some cases reprisals by extremists or forces loyal to the Syrian government.
There was no immediate reaction from Russia, a long-standing arms supplier to Syria and Assad ally, whose ties with Washington are at their lowest since the end of the Cold War.
But Russian President Vladimir Putin spoke with UN Secretary General Ban Ki-moon on Monday saying all air strikes on ISIL bases inside Syria "should not be carried out without the agreement of the government of Syria", the Kremlin said in a statement early Tuesday, before the attacks began.
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 ISIL.
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 ISIL 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.