UAE, Bahrain, Qatar and Jordan played roles as part of US-led attack on militants in Syria
The United States and its Arab allies bombed Syria for the first time on Tuesday, killing scores of ISIL fighters and members of a separate Al Qaeda-linked group, opening a new front against militants by joining a three-year-old civil war.
US Central Command said Bahrain, Jordan, Qatar, Saudi Arabia and the United Arab Emirates participated in or supported the strikes against ISIL targets around the eastern cities of Raqqa, Deir Al Zor, Hasakah and Albu Kamal.
Warplanes and ship-launched Tomahawk cruise missiles struck "fighters, training compounds, headquarters and command and control facilities, storage facilities, a finance centre, supply trucks and armed vehicles," it said.
Washington also said US forces had acted alone to launch eight strikes in another area of Syria against the "Khorasan Group", an Al Qaeda unit US officials have described in recent days as posing a threat similar to that from ISIL.
The Syrian Observatory for Human Rights, which monitors the war in Syria, said at least 70 ISIL fighters were killed in strikes that hit at least 50 targets in Raqqa and Deir Al Zor and Hasakah provinces in Syria's east.
It said at least 50 fighters and eight civilians were killed in strikes targeting Al Qaeda's Syrian affiliate, the Nusra Front, in northern Aleppo and Idlib provinces, apparently referring to the strikes the Americans said targeted Khorasan.
The Observatory said most of the Nusra Front fighters killed were not Syrians.
The air attacks fulfil President Barack Obama's pledge to strike in Syria against ISIL, a Sunni Muslim group that has seized swathes of Syria and Iraq, imposing a mediaeval interpretation of Islam, slaughtering prisoners and ordering Shi'ites and non-Muslims to convert or die.
In a sign of how ISIL's rise has blurred lines in Middle East conflicts, the Syrian government said Washington had informed it hours before the strikes in a letter from Secretary of State John Kerry sent through his Iraqi counterpart.
A Syrian foreign ministry statement refrained from criticising the US-led action and said Damascus would continue to attack ISIL and was ready to cooperate with any international effort to fight terrorism.
Only a year ago Washington was on the verge of bombing the Syrian government to punish it for using chemical weapons, before Obama cancelled those strikes at the last minute.
A Syrian analyst interviewed on tightly-controlled state TV said the air strikes did not amount to an act of aggression because the government had been notified in advance.
"This does not mean we are part of the joint operations room, and we are not part of the alliance. But there is a common enemy," said the analyst, Ali Al Ahmad.
Residents reached by telephone in Raqqa, ISIL's de facto capital in eastern Syria, said people were fleeing for the countryside after the bombs started falling overnight.
ISIL vowed revenge against the United States.
"These attacks will be answered," an ISIL fighter told Reuters by Skype from Syria, blaming Saudi Arabia's ruling family for allowing the strikes to take place.
The Sunni fighters, who have proclaimed a caliphate ruling over all Muslims, shook the Middle East by sweeping through northern Iraq in June. They alarmed the West in recent weeks by beheading two US journalists and a British aid worker, raising fears that they could attack Western countries.
The strikes took place hours before Obama goes to the UN General Assembly in New York where he will try to rally more nations behind his drive to destroy ISIL. The White House said he would make a statement before setting off.
The action pitches Washington for the first time into the three-year-old Syrian civil war, which began with "Arab Spring" democracy protests but descended into a sectarian conflict that has killed 200,000 people, displaced millions and drawn in proxy forces backed by countries across the region.
US forces have previously hit IS targets in Iraq, where Washington supports the government, but had held back from a military engagement in Syria, where Obama still calls for the downfall of President Bashar Al Assad. Washington has said it would not coordinate operations against Islamic State with Assad's government.
ISIL's Sunni fighters, now equipped with US weapons seized during their advance in Iraq, are among the most powerful opponents of Assad, a member of a Shi'ite-derived sect. They are also battling against rival Sunni groups in Syria, against the Shi'ite-led government of Iraq and against Kurdish forces on both sides of the border.
In recent days they have captured villages from Kurds near Syria's Turkish border, sending nearly 140,000 refugees across the frontier since last week. The United Nations said it was bracing for up to 400,000 people to flee.
Washington wants to defeat the fighters without helping Assad. Its Sunni Arab allies oppose Assad and his sponsor Iran.
The Western-backed Syrian opposition, which is fighting against both Assad and ISIL, welcomed the air strikes which it said would help defeat Assad.
The main Syrian Kurdish party welcomed the US action and said it wanted to coordinate action against ISIL.
The targets included Raqqa city, the main headquarters in Syria of IS fighters who have proclaimed a caliphate stretching from Syria's Aleppo province through the Tigris and Euphrates river valleys to the outskirts of Baghdad.
"There is an exodus out of Raqqa as we speak," a resident said by phone. "It started in the early hours of the day after the strikes. People are fleeing towards the countryside."
Photographs taken in Raqqa showed wreckage of what IS fighters said was a drone that had been shot down. Pieces of the wreckage, including what appeared to be part of a propellor, were shown loaded into the back of a van.
A video posted online, filmed through night-vision apparatus, showed lights from jets flying overhead firing a stream of projectiles at the ground. It was not clear where or when the video was filmed.
Jordan, confirming its participation, said its air force had bombed "targets that belong to some terrorist groups that sought to commit terrorist acts inside Jordan." It did not say where.
Israel shot down a Syrian jet in air space it controls, but there was no sign the incident was linked to the US action.
US officials and the Syrian Observatory 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.
Residents in Raqqa had said last week that ISIL was moving underground after Obama signalled on September 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 crucial for the credibility of the American-led campaign. With the backing of Jordan and the Gulf monarchies, Washington has the support of Sunni states hostile to Assad. It has not, however, won support of Assad himself or his main regional ally, Shi'ite Iran.
None of Washington's traditional Western allies has so far joined the campaign in Syria. Britain, which joined the United States in war in Iraq and Afghanistan last decade, said it was still considering its options. France has struck IS in Iraq but not in Syria, citing legal constraints.
A militant group which kidnapped a Frenchman in Algeria on Sunday has threatened to kill him unless Paris halts intervention in Iraq.
NATO ally Turkey, which is alarmed by ISIL but also worried about Kurdish fighters and opposed to any action that might help Assad, has refused a military role in the coalition.
Assad's ally Russia, whose ties with Washington are at their lowest since the end of the Cold War, said any strikes in Syria are illegal without Assad's permission or a UN Security Council resolution, which Moscow would have the right to veto.
Obama backed away from getting involved in Syria's civil war a year ago after threatening air strikes against Assad's government over the use of chemical weapons. The rise of ISIL prompted him to change course and take action against Assad's most powerful opponents.
Washington says it hopes to strengthen a moderate Syrian opposition to fill the vacuum so that it can degrade ISIL without helping Assad. But so far, the opposition groups recognised as legitimate by the United States and its allies have been a comparatively weak force on the battlefield.