US and Arab states have carried out the first air strikes against militant group Islamic State (IS) on the territory of Syria, the Pentagon has announced.

According to Rear Admiral John Kirby, fighter and bomber jets and Tomahawk missiles were part of the military action, according to ABC news.

Kirby has refused to elaborate further, "given that these operations are ongoing" and the US is "not in a position to provide additional details at this time."

Tens of jihadists have reportedly died after a first wave of 20 strikes targeting the Syrian town of Raqqa and other points alongside the Syria-Iraq border in the early hours of Tuesday.

More air attacks followed, with some reports putting the number at 59.

NATO countries, Arab nations and other non-members such as Australia are part of the campaign.

Syrian authorities say they were informed of the strikes, even though they had not formally consented them, Al Jazeera reports citing Syria's Foreign Ministry.

Raqqa has been in IS hands since 2013.

Diplomatic sources quoted by US media outlets say military officials from Bahrain, Jordan, Qatar, Saudi Arabia and the United Arab Emirates (UAE) are taking part. So far only Jordan has confirmed.

US media outlets suggest the Arab states have decided to act in a rare show of solidarity against the militants.

Syria's National Coalition, the main opposition alliance, has welcomed the move, but has stressed more than military strikes would be needed to defeat extremism.

IS is now said to have as many as 31 000 fighters in Iraq and Syria, where it controls large chunks of land.

Positions of Al-Qaeda affiliate Al-Nusra Front were also attacked by the air strikes.