The first day of a two-day meeting of the ministers in Brussels was dominated by the issue, with Turkey isolated among the 29 member states because of its incursion against Kurdish fighters it considers "terrorists" but who are key in the fight against the Islamic State (IS) group in Syria.

NATO Secretary General Jens Stoltenberg described discussions as "frank and open" -- euphemisms for sharp discord -- and noted "we have seen disagreements before" but the transatlantic alliance has endured.

He stressed that the ministers agreed on the need to "maintain our unity in the fight against ISIS," referring to the IS group being fought in Syria, Iraq, Afghanistan and elsewhere by a broad international coalition including many NATO members.

Germany presented an idea it floated this week of international troops being deployed to create a security zone in northeast Syria -- a notion that has been met tepidly by allies because of the situation on the ground and the need for a UN mandate.

The top commander of Syria's Kurdish force, Mazloum Abdi, welcomed the proposal, telling journalists in northern Syria that "we demand and agree to this".

- 'Terrible situation' -

But the NATO ministers did not directly embrace the German plan. Stoltenberg said they instead stressed their "broad support... for ways to engage the international community to find a political situation" in northern Syria.

Before the meeting, German Defence Minister Annegret Kramp-Karrenbauer said she and her French and British counterparts believed a Turkish-Russian agreement to jointly patrol a "safe zone" inside northern Syria "does not provide a permanent basis for a political solution".

Belgium's defence minister, Didier Reynders, said of Germany's troops idea: "In principle we are in favour of such an agreement to work together -- but then again, the situation is totally different now" following the Turkey-Russia agreement.

US Defense Secretary Mark Esper, speaking at a think tank conference in Brussels before the NATO meeting, was blunt about Turkey, saying it was "heading in the wrong direction".

"Turkey put us all in a very terrible situation and I think the incursion's unwarranted," Esper said.

He defended the withdrawal of "less than 50" US troops from northern Syria that cleared a path for the Turkish operation, arguing it was the only way to preserve the soldiers' lives, and that in any case he was not "about to start a fight with a NATO ally".

- Trump's eyes on Syria's 'Oil Region!' -

US President Donald Trump on Wednesday summed up American strategy in Syria by saying: "Let someone else fight over this long blood-stained sand."

However on Thursday, he tweeted: "I appreciate what the Kurds have done. Perhaps it is time for the Kurds to start heading to the Oil Region!"

That referred to a non-Kurdish, crude-rich desert zone in Syria's northeast under US control that Trump wants to prevent falling into the hands of the Syrian regime or its Iranian or Russian partners.

Turkey's actions, its rapprochement to Russia and its threat to its European allies in NATO to unleash a wave of refugees if they dared criticise the assault in Syria have unnerved many in the transatlantic alliance.

"When we say we will open the gates, they are up in arms. Don't be up in arms, the gates will be opened when the time comes," Turkish President Recep Tayyip Erdogan said in a televised speech in Ankara.