NATO member Turkey's military operation in northern Syria is to dominate a meeting of North Atlantic Treaty Organisation defence ministers in Brussels on Thursday and Friday.

The issue has become even more controversial after Turkey on Tuesday sealed a deal with Moscow for Russian forces to help clear Kurdish fighters from the "safe zone" it is creating in northern Syria and to patrol the area.

US ambassador Kay Bailey Hutchison said her country hailed the idea launched by German Defence Minister Annegret Kramp-Karrenbauer of international forces creating a security zone in northeastern Syria.

Kramp-Karrenbauer told broadcaster Deutsche Welle on Monday that European forces, including from Germany, could be part of that mission, to resume the fight against the Islamic State (IS) group and "stabilise the region".

Hutchison said the idea "to put together a European group that would be part of a peacekeeping group would certainly be positive".

She added: "If the Turks ask for more help from the international community, I think the Europeans could step forward and I think the German minister along with France and the UK could answer that call and it would be a positive thing."

A pull-out by US forces in northern Syria ordered early this month by President Donald Trump opened the way for Turkey to launch an offensive against Kurdish forces previously allied with the US.

- 'Frank' discussions -

NATO diplomats said there have been "frank" discussions with Turkey on its military operation, and the country was isolated within the organisation.

But, they stressed, Turkey was "too strategically important" for any concrete action against it, and in any case NATO decisions are made unanimously by all 29 members states -- Turkey included.

NATO Secretary General Jens Stoltenberg told reporters Wednesday that it was "absolutely meaningless to hide there are differences regarding north Syria" between Turkey and other member states.

He said the defence ministers' meeting on Thursday would be useful because "the best way to address these differences is to sit together at the table and to discuss and try to find a way forward".

He was cautious on the Turkey-Russia deal worked out, saying it was "a bit early to judge the consequences". But he said NATO welcomed any efforts that de-escalated the conflict in Syria.

The transatlantic alliance is not operationally active in Syria, but many of its member states are involved in the anti-IS fight there and in other countries, including in Afghanistan where NATO is deployed.

Stoltenberg said that Germany's idea of a secure zone in Syria patrolled by international force was to be discussed during the NATO ministers' meeting.

However diplomats said they had no details about it, with some hinting it would get short shrift.

A French source with knowledge of discussions around the idea said Tuesday the initiative was "not coordinated" with Germany's partners and in any case ran contrary to "the dynamics" on the ground in Syria.