The EU's diplomatic chief on Thursday backed a Dutch plan for a no-fly zone over northwest Syria's Idlib, but warned the bloc had no power to make it happen.
Josep Borrell, the EU's foreign affairs high representative, said he supported the idea of creating a "buffer zone" to stop attacks on civilians in the rebel-held area.
EU foreign ministers are meeting in Zagreb for talks on the Syrian civil war and the growing diplomatic spat with Turkey, which has sent thousands of migrants to the Greek border raising fears of a return of Europe's 2015-16 migrant crisis.
Turkey called last week for a no-fly zone over Idlib province to protect civilians from aerial bombardment by Russian-backed regime forces.
Borrell supported Dutch Foreign Minister Stef Blok's call made on Wednesday for a halt to air attacks on Idlib, through a no-fly zone for Syrian warplanes and helicopters.
"If we want a ceasefire, if we want to end the hostilities and end the bombing which affects cruelly civilians and civilian facilities, we should create a buffer zone in the airspace of the fighting," Borrell told reporters in Zagreb.
He warned that the EU -- which has no serious military resources of its own to call on -- could not instigate it alone.
"The European Union cannot decide to have a buffer zone in Syria. We would like to speak the language of power but for the time being we cannot decide this by our own," he said, but argued the bloc could influence the UN and NATO to act.
Blok said the ideal solution would be a UN Security Council resolution backing a no-fly zone, but with Assad ally Russia holding a veto this looks unlikely.
Instead, Blok said, willing EU countries could possibly broker an agreement for a zone with Turkey and Moscow.
German Foreign Minister Heiko Maas also lent his support, saying the safety of Idlib civilians demanded "a zone with security guarantees" given by Russia.
While EU ministers gathered in Croatia, Russian President Vladimir Putin met his Turkish counterpart Recep Tayyip Erdoğan in Moscow after a surge in fighting in Syria raised fears of their armies clashing.
Intense fighting has killed dozens of Turkish soldiers in Idlib in recent weeks, as Ankara for the first time launched a direct offensive against Assad's forces.
The push by Damascus to take Idlib has forced almost a million civilians to flee their homes and forced Turkey to open the border with Greece to refugees and migrants.
