The United States and its Arab allies bombed Syria for the first time on Tuesday, killing scores of Islamic State fighters and members of a separate al Qaeda-linked group, opening a new front against militants by joining Syria's three-year-old civil war.

In a remarkable sign of shifting Middle East alliances, the attacks took place with no objection - and even signs of tacit approval - from President Bashar al-Assad's Syrian government, which said Washington had notified it in advance.

U.S. Central Command said Bahrain, Jordan, Qatar, Saudi Arabia and the United Arab Emirates participated in or supported the strikes against Islamic State targets. All are countries deeply hostile to Assad but now fearful of the fighters that have emerged out of the anti-Assad rebellion they backed.

Warplanes and ship-launched Tomahawk cruise missiles struck "fighters, training compounds, headquarters and command and control facilities, storage facilities, a finance center, supply trucks and armed vehicles," CentCom said.

Washington also said U.S. forces had acted alone to launch eight strikes in another area of Syria against the "Khorasan Group", an al Qaeda unit U.S. officials have described in recent days as posing a threat similar to that from Islamic State.

The Syrian Observatory for Human Rights, which monitors the war in Syria, said at least 70 Islamic State 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 fulfill President Barack Obama's pledge to strike in Syria against Islamic State, a Sunni Muslim group that has seized swathes of Syria and Iraq, slaughtering prisoners and ordering Shi'ites and non-Muslims to convert or die.

It remains to be seen how effective air strikes can be against Islamic State in Syria, where Washington does not have a strong ally to fight the group on the ground.

U.S. officials, speaking on condition of anonymity, said Washington and its Arab allies would wage a persistent air campaign in Syria, with the pace of future strikes to depend on finding available targets.

"COMMON ENEMY"

In a sign of how Islamic State'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 criticizing the U.S.-led action. It said Damascus would continue to attack Islamic State 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 canceled those strikes at the last minute.

Tightly-controlled Syrian state TV interviewed an analyst who said the air strikes did not amount to an act of aggression because the government had been notified.

"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, Islamic State's de facto capital in eastern Syria, said people were fleeing for the countryside after the bombs started falling overnight.

Islamic State vowed revenge against the United States.

"These attacks will be answered," an Islamic State 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 killing two U.S. journalists and a British aid worker, raising fears that they could attack Western countries.

The strikes took place hours before Obama goes to the U.N. General Assembly in New York where he will try to rally more nations behind his drive to destroy Islamic State. The White House said he would make a statement before setting off.

PITCHED INTO CIVIL WAR

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.

The Syrian military pressed its campaign against the rebels unabated on Tuesday, shelling and carrying out air strikes in the southern province of Deraa and the outskirts of Damascus, as well as Raqqa and Idlib provinces, the Observatory said. Rebel and loyalist forces fought in the northern city of Aleppo.

U.S. forces have previously hit Islamic State 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 Assad. Washington has said it would not coordinate action against Islamic State with Assad's government.

Islamic State's Sunni fighters, equipped with U.S. weapons seized during their advance in Iraq, are among the most powerful opponents of Assad, a member of a Shi'ite-derived sect and ally of Shi'ite Iran. 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.

The Western-backed Syrian opposition and Syrian Kurdish groups, who are fighting against both Assad and Islamic State, welcomed the air strikes and said they need more support.

The targets included Raqqa city, the main headquarters in Syria of Islamic State 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."

The city's two-storey main administrative building had been hit by four rockets, which were so precise that nearby buildings were not damaged, said the resident, named Abo Mohammed. He said hundreds of fighters, who had been visible in the streets controlling traffic and security, had now vanished.

ARAB PRESENCE KEY, TRADITIONAL ALLIES ABSENT

The presence 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.

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 Islamic State in Iraq but not in Syria, citing legal constraints.

NATO ally Turkey, which is alarmed by Islamic State 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 U.N. Security Council resolution, which Moscow would have the right to veto.

(Additional reporting by Alexander Dziadosz and Mariam Karouny in Beirut, Lesley Wroughton, Steve Holland and Matt Spetalnick in Washington; Writing by Peter Graff; Editing by Paul Taylor and Janet McBride)