In this picture downloaded from the US Navy website and taken on Tuesday, an F/A-18C Hornet prepares to launch from the deck of the aircraft carrier USS George H.W. Bush in the Arabian Gulf, to conduct strike missions against Islamic State group targets. Photo: AFP

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 from President Bashar al-Assad's Syrian government, which said Washington had notified it in advance.

The US said Bahrain, Jordan, Qatar, Saudi Arabia and the United Arab Emirates participated in or supported the strikes against IS targets.

US President Barack Obama on Tuesday vowed to continue the fight against IS fighters following the US-led airstrikes in Syria, and pledged to build even more international support for the effort.

Obama said the strength of the coalition, now at more than 40 countries, shows America is not alone in the fight against such militants.

"America is proud to stand shoulder to shoulder with these nations on behalf of our common security," he said. "The strength of this coalition makes it clear to the world that this not America's fight alone."

Obama also said he would meet with Iraqi Prime Minister Haider al-Abadi and "friends and allies" at the United Nations to continue building support for the coalition.

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," US Central Command 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 IS.

The Syrian Observatory for Human Rights, which monitors the war in Syria, said at least 70 IS 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 Obama's pledge to strike in Syria against IS, 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 airstrikes can be against IS in Syria, where Washington does not have a strong ally to fight the group on the ground.

US 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.

In a sign of how IS'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 US-led action. It said Damascus would continue to attack IS 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.

Syrian state TV interviewed an analyst who said the airstrikes 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.

IS vowed revenge against the US.

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 US journalists and a British aid worker, raising fears that they could attack Western countries.