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.

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.

The Syrian government has not formally consented to the air strikes on its territory. However, it says it was informed before they took place.

Reports indicate many of the strikes hit Raqqa, an IS stronghold in eastern Syria the group captured in 2013.

Social media reports said sites hit include the governor's house, the Equestrian Club and a hospital.

F-22 fighters - equipped with stealth capabilities - were among the warplanes deployed in the strike, US media reported.

'Bombing without consent'
US media reports said Bahrain, Jordan, Qatar, Saudi Arabia and the United Arab Emirateswere among those involved alongside the US. Jordan confirmed it had taken part but gave no details on its role.

Analysts say it is significant that countries with a Sunni majority, such as Saudi Arabia, appear to be among those supporting US efforts against IS.

IS members are jihadists who adhere to an extreme interpretation of Sunni Islam and consider themselves the only true believers.

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.

Syria's foreign ministry said in a statement that its UN envoy was informed about the strikes.

"The American side informed Syria's permanent envoy to the UN that strikes will be launched against [Islamic State] in Raqqa," the statement, carried on Syrian state media, said.

The US and allies including the UK have ruled out co-operating against IS with Syria's President Bashar al-Assad, whom they accuse of responsibility for huge numbers of civilian deaths during Syria's civil war.

'Safe havens'
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, which the CIA says could have as many as 31,000 fighters in Iraq and Syria, has executed captive soldiers, aid workers and journalists, and threatened the mass killing of Iraqi religious minorities.

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".

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 Barack Obama]", Rear Adm Kirby said.

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.