An Islamic State fighter has vowed retribution for US-led air strikes against the group, after the attack launched with Arab allies killed dozens of militants overnight.
"These attacks will be answered", the Isis militant told the Reuters news agency, while blaming the "sons of Saloul" - a derogatory term for Saudi Arabia's ruling family - for allowing the strikes to take place.
The Pentagon spokesman Rear Adm John Kirby said fighter jets, bombers and Tomahawk cruise missiles fired from ships in the region during the strikes, most of which are reported to have hit the Isis stronghold Raqqa overnight.
In a statement, the US military said it had "destroyed or damaged multiple (Isis) targets" around the cities of Raqqa, Deir al-Zor, Hasakah and the border town of Albu Kamal.
Britain was not involved in the strikes but the Ministry of Defence (Mod) said discussions on whether to join the strikes in Syria are “ongoing” and "no decision has been taken to our involvement".
The Prime Minister David Cameron has said the UK supports the air strikes and will be holding talks at the United Nations over the next few days to discuss what the UK can do to tackle Isis.
The Syrian Foreign Ministry claimed the US informed Damascus before its military and other countries launched the first air strikes.
Syrian state television reported on Tuesday that Washington informed Syria's United Nations envoy before the attacks began, quoting Syria's Foreign Ministry as the source. However, it did not formally consent to them taking place in its territory.
A statement from the Ministry claimed that: “the American side informed Syria's permanent envoy to the UN that strikes will be launched against the Daesh (an Arabic name for Isis) organisation in Raqqa”.
The US says Bahrain, Qatar, Saudi Arabia, Jordan and the United Arab Emirates are involved in the air strikes - although only Jordan has directly confirmed its involvement.
The US military are also understood to have targeted a second al-Qaeda-linked militant group - the Khorasan Group - to disrupt the threat of "imminent attack plotting against the United States and Western interests".
A statement by the Pentagon said: "Separately, the US has also taken action to disrupt the imminent attack plotting against the US and Western interests conducted by a network of seasoned al-Qaeda veterans - sometimes referred to as the Khorasan Group - who have established a safe haven in Syria to develop external attacks, construct and test improvised explosive devices and recruit Westerners to conduct operations.
"These strikes were undertaken only by US assets."
US officials said the air strikes began at about 1.30am British time and finished about 90 minutes later. The operation was expected to continue for several more hours.
The strikes come after President Barack Obama stressed it would not coordinate with the government of President Bashar al-Assad in any way in its fight against Isis.
The strikes were carried out by manned air force and navy aircraft and the Tomahawk missiles were launched from US ships in the northern Persian Gulf and the Red Sea, according to the Associated Press. The aircraft carrier USS George HW Bush is in the Gulf.
The attack formed part of the expanded military campaign authorised by President Obama, who has vowed to "degrade and destroy" Isis militants.
The group has advanced across large swathes of Syria and northern and western Iraq, committing a spate of atrocities in its wake and beheading Western journalists and an aid worker.
The US military has already launched 190 air strikes within Iraq since August, initially focusing specifically on attacks to protect American interests and staff, assist Iraqi refugees and secure critical infrastructure.
Last week, as part of the newly-expanded campaign, the US began going after militant targets across Iraq.
Military officials have said the US would target militants' command and control centres, re-supply facilities, training camps and other key logistical sites.
Last week General Martin Dempsey told senators: “We will be prepared to strike Isis targets in Syria that degrade Isis's capabilities.
The chairman of the US Joint Chiefs of Staff said the strikes would be a "persistent and sustainable campaign" as opposed to a "shock-and-awe" campaign.
The US has been increasing its surveillance flights over Syria to gain more intelligence on potential targets and militant movements.
Military leaders say about two-thirds of the estimated 31,000 Islamic State militants were in Syria.
Isis has been threatening retribution against the most recent US action. Its spokesman, Abu Mohammed al-Adnani, said in a 42-minute audio statement released on Sunday that the fighters were ready to battle the US-led military coalition and called for attacks at home and abroad.
The group has also released two ‘Flames of War’ propaganda videos showing tanks being hit, wounded US soldiers and men on their knees as they are about to be executed, in an apparent warning to America over its targeting of Isis.
Additional reporting by agencies
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