US warplanes have begun attacking Islamic State targets in Syria, in an assault which began with a strikes by sea-launched cruise missiles.
The Pentagon said the strikes were being carried out by the US and by “partner nations”, and were still ongoing.
“We are not in a position to provide additional details at this time,” the Pentagon tweeted.
The raids involved fighter jets, bombers, and Tomahawk cruise missiles.
Reports said the attacking aircraft also included F-22 Raptors – the first time the stealth fighter jets had been used in combat.
US media reports said at least 20 targets had been hit, mostly in and around the Islamic State-held city of Raqqa and nearby Tabqa.
A US official told the Reuters news agency that Saudi Arabia, Jordan, Bahrain and the UAE were involved in the attacks, but their exact roles were unclear.
US military & partner nation forces have begun striking ISIL targets in Syria using mix of fighters, bombers and Tomahawk missiles.
— Rear Adm. John Kirby (@PentagonPresSec) September 23, 2014
Raqqa, in Syria’s north, has been considered a stronghold for Islamic State and other militant groups since the insurgents seized control of much of the city in March 2013.
Syrian media activists reported explosions in Raqqa and Idlib governorates.
“The international coalition struck IS positions, bases and checkpoints in Raqqa city, and near the towns of Tabqa, Ain Eissa and Tal Abyad, killing jihadists,” the Syrian Observatory for Human Rights said.
Videos posted to social media, which have not been verified, showed huge explosions lighting up the night sky.
US Rear Admiral John Kirby confirmed the air strikes, tweeting that: “US military & partner nation forces have begun striking ISIL targets in Syria using mix of fighters, bombers and Tomahawk missiles”.
The decision to conduct the air strikes was undertaken earlier on Monday by the head of US Central Command, General Lloyd Austin, “under authorisation granted him by the commander in chief,” Rear Admiral Kirby said.
Islamic State militants have recently made territorial gains in Syria, with thousands of refugees fleeing as the Sunni militants closed in on a Kurdish town.
Syria’s foreign ministry said Damascus had been informed by Washington about the air strikes.
“The Americans had informed the Syrian representative at the United Nations that strikes would be carried out against the terrorist IS organisation in Raqqa,” state television reported the ministry as saying.
US warplanes began hitting Islamic State targets in Iraq early last month.
Militants raping women, forcing girls into marriage: Kerry
Earlier, US secretary of state John Kerry said Islamic State militants were committing horrific abuses against women in both Syria and Iraq.
“ISIL has savagely raped and kidnapped countless Iraqi and Syrian women, it has forced young girls into marriages with ISIL fighters,” he said.
“It has awarded young women to jihadists as a reward, has abducted Yazidi and Christian women and sold them off as slaves to jihadists.
“This is who ISIL is. And what we’re doing here is saying who we are and what matters to us.
“I wish it were otherwise but barbarity is obviously not new to the world. Evil has been with us for a long time too, but we don’t stop fighting it.”