By Phil Stewart and Tom Perry

WASHINGTON/BEIRUT (Reuters) - The United States and its Arab allies bombed militant groups in Syria for the first time on Tuesday, killing scores of Islamic State fighters, members of a separate al Qaeda-linked group and opening a new front amid shifting Middle East alliances.

The attacks encountered no objection, and even signs of tacit approval, from President Bashar al-Assad's Syrian government, which said Washington had warned Damascus in advance.

Saudi Arabia, Qatar, Jordan, Bahrain and the United Arab Emirates participated in or supported the strikes against Islamic State targets, U.S. Central Command said. The countries are hostile to Assad but now fear the fighters who emerged from the rebellion they backed in Syria's 3-year-old civil war.

U.S. President Barack Obama said in a televised statement that the breadth of the coalition, including the five Arab states, showed the United States was not alone in its second campaign of air strikes. Since Aug. 8, U.S. air strikes have hit militant targets in Iraq, where Washington supports the government, but had held back from a military engagement in Syria, where it is at odds with Assad.

The White House said some of the strikes in Syria had targeted an al Qaeda affiliate known as the Khorasan group, which it said had been plotting an imminent attack either in the United States or in Europe.

"Once again, it must be clear to anyone who would plot against America and do Americans harm that we will not tolerate safe havens for terrorists who threaten our people," Obama said before leaving the White House for the United Nations.

In New York, Obama planned more talks to enlarge the alliance against extremist groups that emerged and gained power while trying to topple Assad. In a reversal, Turkey indicated Tuesday it would provide military or logistical backing.

"We will give the necessary support to the operation," President Tayyip Erdogan told Turkish broadcaster NTV.

Story continues

The NATO ally, which is alarmed by Islamic State but also worried about Kurdish fighters and opposed to any action that might help Assad, had refused a military role in the coalition while 46 of its citizens were held by the group in Iraq. Turkey is home to a major U.S. base in Incirlik, which officials said has not been used so far in the strikes in Iraq or Syria.

Warplanes and ship-launched Tomahawk cruise missiles struck dozens of targets including fighters, training compounds, headquarters and command and control facilities, storage sites, a finance center, trucks and armed vehicles, CENTCOM said.

"I can tell you that last night's strikes were only the beginning," said Rear Admiral John Kirby, a U.S. Defense Department spokesman. He called the overnight attacks "very successful" but gave few other details.

The U.S.-led coalition launched 16 airstrikes on Islamic State across Syria, CENTCOM said in a statement later on Tuesday.

Washington also said U.S. forces had acted alone to launch eight strikes in northeastern Syria on what they called the Khorasan group.

Militants on social media mourned Khorasan's reputed leader, Mohsin al-Fadhli, a former associate of al Qaeda founder Osama bin Laden. U.S. officials have not confirmed his death.

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 the provinces of Raqqa, Deir al-Zor and Hasakah.

It said at least 50 fighters and eight civilians were killed in strikes on the Khorasan group, which was thought to operate in Syria with the Nusra Front, another al Qaeda offshoot that opposes Islamic State.

The air attacks fulfill 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 in Syria, where Washington lacks a strong ally to fight the group on the ground. The militants vowed reprisals, and an allied group is threatening to kill a French hostage captured in Algeria.

In Washington, the Department of Homeland Security and the FBI said they had alerted law enforcement agencies to a threat from Syrian-based al Qaeda operatives "nearing the execution phase for an attack in Europe or the homeland."

'COMMON ENEMY'

In a sign of how Islamic State's rise has blurred conflict lines, the Pentagon said the U.S. ambassador to the United Nations, Samantha Power, had told Syria's envoy in advance but there was no communication between the countries' armed forces.

Syrian state media reported that a senior Iraqi envoy briefed Assad on the next steps and the Syrian leader said he supported any international effort to fight terrorism.

Only a year ago, Washington was on the verge of bombing the Syrian government over the use of chemical weapons, before Obama canceled the 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.

Syria's closest ally, Iran, responded cautiously. President Hassan Rouhani said in New York that without a U.N. mandate or a request from the Assad government, military strikes had no legal standing. Rouhani, however, neither condemned nor endorsed the action.

Officials from both countries later said the United States also told Iran in advance of the air strikes.

Residents of Raqqa, Islamic State's de facto capital in eastern Syria, said by telephone that people were fleeing for the countryside after the bombs fell overnight.

Islamic State vowed revenge against the United States. "These attacks will be answered," a fighter told Reuters by Skype from Syria, blaming Saudi Arabia's ruling family for allowing the strikes to take place.

Saudi Arabia, a U.S. ally, the world's top oil exporter and birthplace of Islam, has funneled cash and arms to Sunni Muslim rebels fighting Assad but has also opposed Islamist militants within the insurgency. Its air force participated in the bombing strikes, a rare foreign sortie for the kingdom that showed how much was at stake.

"Today we face a very dangerous situation where terrorist cells have turned into armies ... that extend to all of Libya, Lebanon, Syria, Iraq and Yemen," Saudi Foreign Minister Prince Saud al-Faisal Al Saud told the Saudi Press Agency.

Islamic State 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 American journalists and a British aid worker, raising fears they could attack Western countries.

The presence of Arab allies in the attacks was crucial for the credibility of the U.S.-led campaign. With the backing of Jordan and the Gulf monarchies, Washington has the support of Sunni states hostile to Assad, a member of a Shi'ite-derived sect.

PITCHED BATTLES

The Syrian civil war began with "Arab Spring" democracy protests in 2011 but has descended into a sectarian conflict that has killed 200,000 people, displaced millions and drawn in proxy forces backed by countries across the region.

Islamic State fighters, equipped with U.S. weapons seized in Iraq, are among the most powerful opponents of Assad. They are also battling rival Sunni groups in Syria, the Shi'ite-led government of Iraq and 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 Western-backed Syrian opposition and Syrian Kurdish groups, which are fighting both Assad and Islamic State, welcomed the air strikes and said they needed more support.

"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-story 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.

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.

Assad's ally Russia, whose ties with Washington are at their lowest since the end of the Cold War, said any strikes in Syria were 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, Roberta Rampton, Lesley Wroughton, Steve Holland, Matt Spetalnick, Doina Chiacu, in Washington; Writing by Peter Graff and Doina Chiacu; Editing by David Stamp, Grant McCool and Peter Cooney)