A boat carrying migrants from Turkey to Greece has sunk with the loss of 25 lives, Turkey's coast guard says.
Fifteen people were rescued after the boat capsized near the Turkish resort of Didim.
Reports suggest Macedonia has set new curbs on Syrian migrants trying to cross the land border from Greece.
Nato is expanding its mission in the Aegean to send patrols to Turkish and Greek territorial waters in the battle to defeat people smugglers.
Secretary General Jens Stoltenberg said in a statement: "The purpose of Nato's deployment is not to stop or push back migrant boats, but to help our allies Greece and Turkey, as well as the European Union, in their efforts to tackle human trafficking and the criminal networks that are fuelling this crisis."
An emergency summit will take place between the EU and Turkey on Monday to try to seek a common approach to handling the flow of arrivals.
A Turkish search and rescue operation was launched after the latest sinking in the Aegean Sea.
At least three children are believed to be among those who died.
The International Organization for Migration (IOM) says at least 321 migrants drowned trying to get to Greece between 1 January and 3 March.
More than 2,000 migrants, most of whom the IOM says are from Syria, Iraq and Afghanistan, are arriving into Greece from Turkey every day hoping to travel further north through Europe.
But some EU countries re-imposed internal border controls and Macedonia sharply reduced the numbers allowed to cross, leading to a build-up on the Greek side of the border.
On Sunday, reports from the border said Macedonia had stopped allowing entry to anyone from areas in Iraq and Syria it did not consider to be active conflict zones.
As a result, anyone from Damascus or Baghdad is not allowed to pass.
Babar Baloch, a spokesman for the UN's refugee agency, called the development "concerning".
He said even some migrants from the Syrian cities of Latakia and Homs were being turned back. While a truce was signed between the Syrian government and rebels in Homs in December, the city remains a scene of devastation.
On Saturday, Apostolos Tzitzikostas, governor of a northern border region, said the number of migrants on the frontier waiting to cross north into Macedonia had grown to at least 13,000.
Mr Tzitzikostas called for a state of emergency to be imposed on the border that could facilitate the delivery of aid.
Last week, European Council President Donald Tusk said he had been told by the Turkish leader Recep Tayyip Erdogan that his country was ready to take back all migrants apprehended in Turkish waters.
In other developments:
- Greek Prime Minister Alexis Tsipras said the build-up of migrants in his country meant there was an "urgent" need for European countries to relocate them
- German Chancellor Angela Merkel has called on Greece to complete accommodation for migrants "at lightning speed" as it was already behind schedule
- Austria's Chancellor Werner Faymann said Germany must set a clear limit this year to how many migrants it is willing to accept, to "finally create clarity or else refugees will continue to head off in the direction of Germany"
- Greece's coastguard rescued 368 people from dinghies as they travelled to the Greek island of Lesbos from Turkey
A note on terminology: The BBC uses the term migrant to refer to all people on the move who have yet to complete the legal process of claiming asylum. This group includes people fleeing war-torn countries such as Syria, who are likely to be granted refugee status, as well as people who are seeking jobs and better lives, who governments are likely to rule are economic migrants.