NATO will take any request to help with the refugee crisis "very seriously", chief Jens Stoltenberg said Tuesday after Germany and Turkey sought the alliance's assistance in combatting human smugglers.

Defence ministers from the 28-nation group will discuss the issue at a meeting in Brussels Wednesday and Thursday when they review NATO's response to a more assertive Russia and the security threat posed by the Syria crisis.

"I think we will take very seriously the request from Turkey and other allies to look into what NATO can do to help them cope and deal with the crisis and all the challenges they face, not least in Turkey, " Stoltenberg told a news conference.

German Chancellor Angela Merkel said on a visit to Ankara on Monday that Turkey and Germany would ask NATO to help police the Turkish coast to prevent smugglers from packing migrants into boats for the perilous crossing to Greece.

Turkey -- the only Muslim-majority nation in NATO and one of its largest armies -- was the main gateway for the more than one million migrants and refugees who crossed into Europe last year.

The problem shows no sign of slowing -- more than 70,000 made the dangerous crossing from Turkey to Greece in January, with over 400 dying, according to the International Organization for Migration (OIM), and there are fears hundreds of thousands more could follow as the fighting in Syria intensifies.

- 'Desperate situation' -

Former Norwegian premier Stoltenberg said that Russian involvement in Syria was "undermining" peace efforts and "making a desperate situation worse" as more refugees fled the fighting.

"Calm and easing tensions is more important than ever," he added.

As the crisis in Syria deepens, some NATO allies are wary of getting sucked into a conflict which defies solution, especially as Assad's forces now retake ground in a major Russian-backed offensive.

Diplomatic sources said the German-Turkish proposal had come as a surprise.

Greece, whose relations with its NATO partner Turkey are strained over a host of issues, expressed scepticism and contacted Berlin Tuesday to insist on safeguards if such a mission went ahead.

A Greek government spokesman in Athens said Greek Prime Minister Alexis Tsipras had told German Chancellor Angela Merkel "that any involvement by the alliance must be confined solely to the Turkish coast and guarantee Greek sovereign rights."

US ambassador to NATO Douglas Lute told a separate news briefing that such a German-Turkish request would not be unusual and member states regularly asked for help.

At the same time, Lute said the European Union had the "primary responsibility" for managing the migrant crisis.

"This is fundamentally an issue which shld be addressed a couple of miles (kilometres) from here in the EU," he said, referring to the fact that Brussels is headquarters for the bloc.

- EU backing -

The European Union meanwhile said it would welcome any extra assistance in dealing with a crisis which has put the 28-nation bloc under huge strain.

"Of course it is for NATO to take a decision on the opportunity and modalities of the eventual involvement," European Comission spokesman Margaritis Schinas told a news briefing.

"We welcome all discussions on potential measures which could contribute to addressing the refugee crisis, save lives at sea and improve the management of migratory flows and borders."

Stoltenberg said that in December, NATO agreed a package of measures to reassure and support Turkey after Russia launched its air campaign against rebels in Syria seeking to oust long-time Moscow ally President Bashar al-Assad.

This package included deploying AWACS surveillance aircraft over Turkey, air policing and an increased maritime presence, and was agreed after Turkey shot down a Russian fighter jet in late November for violating its airspace.

Since then, the United States has also requested that its allies approve deploying NATO AWACS to help the US-led coalition fighting against Islamic State jihadis in Syria and Iraq.

Stoltenberg said ministers would discuss a "backfilling" proposal whereby NATO allies would deploy AWACS to take over US missions elsewhere, thereby freeing up American planes for anti-IS missions.

He stressed it was "important to understand that it is not NATO that is going to go into any combat role."