The US and allies said to include Arab nations have launched the first air strikes against Islamic State (IS) militants in Syria, the Pentagon says.
Spokesman Rear Adm John Kirby said fighter and bomber jets and Tomahawk missiles were used in the attack.
The strikes were expected as part of President Barack Obama’s pledge to “degrade and destroy” IS, which has taken huge swathes of Syria and Iraq.
The US has already launched 190 air strikes in Iraq since August.However, Monday’s action expands the campaign against the militant group across the national border into Syria.
Islamic State – also known as Isis, or Isil – has taken control of a vast area between Syria and Iraq, imposed a harsh brand of Islam, and declared a caliphate.
The group has executed captive soldiers, aid workers and journalists, and threatened the mass killing of religious minorities in Iraq.
The BBC’s Barbara Plett Usher in Washington DC says the strikes in Syria differ from those in Iraq: while the government of Iraq invited US intervention against the IS militants, the Syrian government has not done so.
That puts the US in the position of bombing an Arab country without its consent, our correspondent reports.
The US and allies including the UK have ruled out co-operating against IS with Syria’s President Bashar al-Assad, who they accuse of responsibility for huge numbers of civilian deaths during Syria’s civil war.
‘Safe haven’ plan
On Monday, Rear Adm Kirby confirmed the strikes, saying “US military and partner nation forces” were undertaking military action in Syria but did not give details.
“Given that these operations are ongoing, we are not in a position to provide additional details at this time,” he said in a statement.
Last week US Secretary of Defense Chuck Hagel told a House of Representatives committee that US Central Command (Centcom), which covers the Middle East, had developed a plan to attack IS “safe havens” in Syria, including “command-and-control, logistics capabilities and infrastructure”.
He said Chairman of the Joint Chiefs of Staff Gen Martin Dempsey, the highest-ranking uniformed military officer in the nation, had approved the plan and briefed the president.
The decision to strike in Syria on Monday was made by Gen Lloyd Austin, the head of Centcom, “under authorisation granted him by the commander in chief [President Back Obama]”, Rear Adm Kirby said.
The BBC has learned that Arab nations were among those involved in the strikes in Syria, but does not have confirmation on which states took part.
In a nationally televised speech outlining his strategy against IS earlier this month, Mr Obama said that any group that threatened America would “find no safe haven”, including inside Syria.
Mr Obama says the strikes were within his power as commander in chief, but has asked Congress to authorise a separate mission to train and arm Syrian rebels fighting against IS.
Congress voted in favour of that measure last week.