The United States and Arab allies bombed Syria for the first time on Tuesday, killing dozens of Islamic State fighters and members of a separate al Qaeda-linked group, pursuing a campaign against militants into a war at the heart of the Middle East.
"I can confirm that U.S. military and partner nation forces are undertaking military action against (Islamic State) 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.
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 around the eastern cities of Raqqa, Deir al-Zor, Hasakah and Albu Kamal.
Targets included "fighters, training compounds, headquarters and command and control facilities, storage facilities, a finance center, supply trucks and armed vehicles," it said.
Separately, U.S. forces acting alone launched strikes in another area of Syria against an al Qaeda-linked group, the Nusra Front, to "disrupt imminent attack" against U.S. and Western interests by "seasoned al Qaeda veterans", CentCom said.
The Syrian Observatory for Human Rights, which monitors the war in Syria, said at least 20 Islamic State fighters were killed in strikes that hit at least 50 targets in Raqqa and Deir al-Zor provinces in Syria's east.
It said strikes targeting the Nusra Front in the northern provinces of Aleppo and Idlib had killed at least 50 fighters and eight civilians. The Nusra Front is al Qaeda's official Syrian wing and Islamic State's rival. The Observatory said most of the fighters killed there 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, imposing a mediaeval interpretation of Islam, slaughtering prisoners and ordering Shi'ites and non-Muslims to convert or die.
Islamic State vowed revenge.
"These attacks will be answered," an Islamic State fighter told Reuters by Skype from Syria, blaming the "sons of Saloul" - a derogatory term for 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 then alarmed the West in recent weeks by beheading 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 action pitches Washington for the first time into the three-year-old Syrian civil war, which has killed 200,000 people and displaced millions.
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 the United States opposes President Bashar al-Assad.
SYRIAN GOVERNMENT INFORMED
The Syrian government said Washington had informed it hours before the strikes. Secretary of State John Kerry had sent a letter to Damascus through his Iraqi counterpart, it said.
A ministry statement read on state television said Syria would continue to attack Islamic State. It was ready to cooperate with any international effort to fight terrorism and was coordinating with the government of Iraq.
The United States has previously said it would not coordinate with Assad's government. Washington says Assad must leave power, particularly after he was accused of using chemical weapons against his own people last year.
Islamic State's Sunni fighters, now 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. 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 is determined to defeat the fighters without helping Assad, a policy that requires deft diplomacy in a war in which nearly all the region's countries have a stake.
The Western-backed Syrian opposition, which is fighting against both Assad and Islamic State, welcomed the air strikes which it said would help defeat Assad.
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.
Photographs taken in Raqqa showed wreckage of what the Islamic State fighter said was a drone that had been shot down. Pieces of the wreckage, including what appeared to be part of a propeller, 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 "a number of targets that belong to some terrorist groups that sought to commit terrorist acts inside Jordan," although it did not specify any location.
Israel shot down a Syrian aircraft over air space it controls in the Golan Heights but there was no indication the incident, confirmed by Syria, was linked to the U.S. action.
WEAPONS SUPPLIES, CHECKPOINTS HIT
U.S. 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 Islamic State was moving underground after Obama signaled 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 crucial for the credibility of the American-led campaign. Some U.S. allies in the Middle East are skeptical of how far Washington will commit to a complex conflict set against the backdrop of Islam's 1,300-year-old rift between Sunnis and Shi'ites.
With the backing of Jordan and the Gulf states, Washington has gained the support of Sunni states that are hostile to Assad. It has not, however, won the open support of Assad himself or his main regional ally, Shi'ite Iran.
Some traditional Western allies, including Britain which went to war alongside the United States in Iraq and Afghanistan, have so far stayed out of the campaign.
France has struck Islamic State in Iraq but not in Syria. A Muslim militant group which kidnapped a French national in Algeria on Sunday has threatened in a video to kill him unless Paris halted intervention in Iraq.
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.
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 Islamic State 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 Islamic State without helping Assad. But so far, the opposition groups recognized as legitimate by the United States and its allies have been a comparatively weak force on the battlefield.