The collapse of the IS "caliphate" has left US-backed Syrian rebel forces holding thousands of foreign fighters and many Western countries are reluctant to take them back due to public opposition and fears they could pose a threat.

But James Jeffrey, the US special representative on Syria and Washington's ambassador to the global coalition to defeat ISIS, said that by dodging their responsibility for their citizens, rich countries were risking fresh violence.

"We're talking about countries with per capita incomes about the same as the United States... putting the burden on informal local authorities who are detached from a state structure in the middle of a war zone," Jeffrey told reporters at a meeting of the coalition in Brussels.

"That's a bad decision. Period. If those people break out -- many of them are dangerous -- they will kill people."

Repatriation is a sensitive issue for Western nations such as France and Britain, which have suffered attacks by homegrown extremists and have little interest in seeing more return.

Britain has even gone as far as stripping some former IS fighters and supporters of citizenship, but the US insists they should be brought home to face the courts.

"They should be prevented from killing people and the best way to do that is to bring them back to Europe and to deal with them in the justice systems of the respective countries," Jeffrey said.

"That's what we're doing in the relatively limited number of cases we have. But we are doing it, why can't they?"

As well as opposition to the cost of detaining the fighters, there are also worries that it could prove difficult to successfully prosecute fighters in European courts for crimes committed in Iraq or Syria.

The coalition met on the sidelines of a NATO defence ministers' gathering, and Jeffrey said the new US Acting Defense Secretary Mark Esper wanted allies to help the fight against the last remnants of IS in northeast Syria.

The Syrian civil war is likely to come up when Trump meets President Vladimir Putin of Russia -- the Syrian government's key backer -- at the G20 which starts in Osaka, Japan on Friday.

Jeffrey said the US was in constant contact with Russia over the conflict and had warned Moscow that any use of chemical weapons in the regime's ongoing assault on the city of Idlib could trigger a military response.

"If chemical weapons are used in Idlib, which is always a possibility, we have made it clear that we will take very very firm action," Jeffrey said, pointing out that the US has already carried out strikes in retaliation for Syrian chemical weapons attacks twice.