The US is pressing allies to contribute more to a US-led military campaign against Islamic State that it says must be accelerated, regardless of the fate of diplomatic efforts to end Syria's civil war.

US Defence Secretary Ash Carter is due to start talks on Thursday afternoon in Brussels with more than two dozen defence ministers, including from key ally Saudi Arabia, which has held out the possibility of sending special forces into Syria.

Carter's push came a day after France delivered a rebuke to President Barack Obama, demanding that Washington display a clearer commitment to resolving the crisis in Syria where Russia is tipping the military balance in favour of President Bashar Assad.

The talks take place as US Secretary of State John Kerry leads a diplomatic push in Munich to rescue imperiled peace efforts, which are taking place despite Russian bombing raids to bolster Syrian forces around the city of Aleppo.

Carter, speaking to reporters ahead of his talks, sought to draw a line between military and diplomatic efforts.

"Our focus here is going to be on counter-ISIL and that campaign will go on because ISIL must be defeated, will be defeated, whatever happens with the Syrian civil war," Carter told reporters, using an acronym for Islamic State.

"But it certainly would help to de-fuel extremism if the Syrian civil war came to an end."

The US hopes the face-to-face gathering of coalition defence ministers will allow it to secure more support for a military campaign that aims to recapture the Islamic State strongholds of Raqqa in Syria and Mosul in Iraq.

WARPLANES, TRAINING

Carter plans to offer a long list of required military capabilities -- which, beyond air power, include training Iraqi forces and help with intelligence and surveillance. Carter said countries that cannot contribute militarily can help in other ways, like by choking Islamic State financing.

"This is a terrific opportunity to have the acceleration that the United States and President Obama has committed to reinforced by all of the military members of the coalition," Carter said.

Still, Carter and US defense officials have sought to manage expectations about the talks, since many ministers will not be able to make new commitments without first winning support from their parliaments. The timeline for the campaign to retake Raqqa and Mosul is 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. - Reuters

Carter only said securing Raqqa and Mosul needed to happen "as soon as possible". He also acknowledged the need to grapple with Islamic State's spread beyond Syria and Iraq. -

Nato announced on Thursday it will seek to help slow refugee flows through the Aegean Sea with a maritime mission to target criminal people smuggling networks. - Reuters