The US armed forces chief says Islamic State (IS) militants are being damaged by air strikes in Syria but air power alone is not enough to defeat them.

Gen Martin Dempsey said a political solution and a ground campaign would both be needed in Iraq and Syria.

Gen Dempsey said that up to 15,000 fighters - to be drawn from Syria's moderate opposition - would be needed for a ground force in Syria.

On Friday, the UK agreed to join US-led air strikes on IS in Iraq.

French fighter jets are already taking part in strikes in Iraq with Belgium and the Netherlands each pledging six F-16s planes and Denmark deploying seven.

Speaking at the Pentagon, Gen Dempsey said this week's strikes in Syria had disrupted IS's command, control and logistics capabilities.

He said it would take more than air power to recapture lost territory in Syria and neighbouring Iraq but a ground force did not need to involve US troops.

"In fact, ideally for the kind of issues we are confronting there, the only truly effective force that will actually be able to reject Isil (IS) from within its own population is a force comprised of Iraqis and Kurds and moderate Syrian opposition," he said.

About 40 countries, including several from the Middle East, have joined the US-led coalition against IS.

European countries have so far only agreed to strike targets in Iraq where the government has asked for help.

But US, Saudi and United Arab Emirates aircraft have also attacked IS targets in eastern Syria, including oil installations.

US-led air raids over Syria are now "near continuous", a US official told AFP news agency on Friday.

US Defence Secretary Chuck Hagel told reporters: "Combined with our ongoing efforts in Iraq, these strikes will continue to deny Isil (IS) freedom of movement and challenge its ability to plan, direct, and sustain its operations."

On Friday, Russian Foreign Minister Sergei Lavrov questioned the legality of air strikes in Syria because they were carried out without the approval of Russia's ally, Syrian President Bashar al-Assad.

"It's very important that such co-operation with Syrian authorities is established, even now that it's an accomplished fact," he said on the sidelines of the UN General Assembly in New York.

(BBC)