BRUSSELS: United States (US) Defence Secretary Ash Carter, who has been under pressure to shore up support from Arab allies to fight the militant Islamic State (IS) group, on Thursday welcomed a commitment from Saudi Arabia to expand its role in air strikes against the militants.

At a gathering of more than two dozen defence ministers at Nato headquarters, Saudi Arabia, which has quietly resumed its participation in air strikes in the past few weeks, also renewed the possibility of sending forces into Syria.

"Saudi Arabia's defence minister ... indicated that the Kingdom is reinvigorating its commitment to the coalition air campaign, which is very welcome news, and contributing in other critical ways on the ground," Carter said after the talks in Brussels.

Carter, broadly praising contributions from allies, said the US-led war against IS would be won despite former Cold War foe Russia's role in the civil war in Syria on the side of President Bashar al-Assad's government.

On Wednesday, France delivered a rebuke to President Barack Obama, demanding that Washington show a clearer commitment to resolving the crisis in Syria, where Russia is tipping the military balance in favour of Assad.

Four months of Russian air strikes in Syria have helped Assad claw back territory from rebels fighting government forces, alarming Gulf Arab states who back the insurgents.

Saudi Arabia said it has carried out more than 190 aerial missions in Syria, although it has focused its military efforts over the last year on the conflict in Yemen, where it is leading a coalition of mainly Gulf Arab forces battling Houthi fighters who control Sanaa.

In Munich, US Secretary of State John Kerry was leading a diplomatic push to rescue imperilled peace efforts, which are being held despite Russian bombing raids to bolster Syrian forces around the city of Aleppo.

Carter sought to draw a line between military and diplomatic efforts, saying IS needed to be defeated "whatever happens with the Syrian civil war".

But he also said Russia's bombing of Western-backed opposition fighters could prolong the civil war that helped give rise to IS.

"I'm confident we'll [defeat IS]. The Russians are not helping in that regard, but we'll do it anyway," he told reporters.

Carter offered allies a long list of required military capabilities, which, beyond air power, included training Iraqi forces and help with intelligence and surveillance. Carter said countries that could not contribute militarily could help in other ways, like by choking IS financing.

Saudi Arabia's Brigadier General Ahmed Asseri, a military spokesman, said his country was ready to send troops into Syria if there was a consensus in the coalition. But he declined to elaborate, saying: "It is too early to talk about such options... Today we are talking at the strategic level," Asseri told reporters in Brussels.

US defence officials had sought to manage expectations about the talks, since many ministers would need to win support from their parliaments. Carter said about a third of US allies made commitments that still required parliamentary approval, appearing to leave many pledges in limbo.

Britain's defence minister Michael Fallon said there was "certainly pressure on those that are still on the sidelines". The timeline for the campaign to end IS control of the strategic cities Raqqa and Mosul in Iraq was also unclear.

The head of the US Defense Intelligence Agency cautioned this week that Iraqi forces were unlikely to recapture Mosul this year, despite hopes by Baghdad.

Carter only said securing Raqqa and Mosul needed to happen "as soon as possible". He also acknowledged the need to grapple with IS's spread beyond Syria and Iraq, particularly in Libya.

"Nobody wants to see Libya on a glide slope to the kind of situation that already engulfed Syria and Iraq," he said.