At least 13 migrants, including children, have died after their dinghy and a ferry collided off Turkey, the coastguard there says.
The accident happened close to the port of Canakkale. The boat was on its way to the Greek island of Lesbos.
In a second incident, Greece's coastguard said 26 people were feared missing from a boat, also off Lesbos.
Meanwhile thousands of migrants continue to cross borders in southern Europe, many now passing into Austria.
Austrian officials told the BBC they expected 20,000 over the weekend.
'We lost the children'
Turkey's coastguard said it had raised the alarm after being told that a commercial vessel and a migrant boat had collided off Canakkale.
It said it had sent a helicopter and six rescue boats to the area.
Eight people have been rescued, with one seriously injured. Some reports said four of the dead were children.
One survivor who gave his name as Haseen, told AFP news agency: "It was dark, we saw the ship bearing down on us. We tried to signal with flashlights and cell phones but they did not see us."
The passengers were thrown into the water.
"We lost the children. We could not see them in the dark," Haseen said.
In the second incident, Greece's coastguard said it had rescued 22 people spotted in the water near Lesbos - but a further 26 people were feared missing.
On Saturday, a five-year-old girl died and 13 people were feared missing after their boat also sank off Lesbos.
Trips on makeshift craft between Turkey and Greek islands are frequent and dangerous for migrants, many of whom are fleeing conflict in Iraq, Syria and Afghanistan.
In other developments:
- US Secretary of State John Kerry said the US would increase its worldwide intake of refugees from 70,000 in 2015 to 85,000 next year and 100,000 in 2017
- Polish Prime Minister Ewa Kopacz said her country would only accept a "symbolic" number of refugees
- Croatia is relocating its refugee operation from Tovarnik on the Serbian border to a new tented settlement at Opatovac, with space for up to 5,000 migrants
Meanwhile, thousands of migrants continue to cross borders further north, many of them hoping to reach Germany and Scandinavia.
Austria saw the arrival of at least 10,000 migrants on Saturday and expects double that by the end of Sunday.
Germany said it expected two trains from Austria, carrying 1,000 migrants in total, to cross its border at Freilassing on Sunday evening.
Five more trains, carrying 500 each, are due on Monday. German police said a planned centre for 400 migrants in Wertheim in southern Germany was destroyed in an arson attack on Sunday morning.
Long queues of both people and traffic have been reported at Slovenia's borders with Croatia and Austria amid tighter restrictions.
Austria, Hungary, Slovenia and Croatia have all traded accusations of failure to deal effectively with the crisis over the past few days.
It began after Hungary sealed its border with Serbia five days ago, erecting a razor-wire fence and making it a criminal offence to cross it.
Thousands of migrants then sought the alternative route via Croatia.
Croatia initially welcomed the migrants but then said it was unable to cope with the numbers and moved them over its border with Hungary, which quickly ferried them on to Austria.
Hungary is now rushing to finish a new fence along its Croatian border, but it has now reopened its Serbian frontier, albeit with strict controls.
Hungary also plans to build a fence along its Romanian border, which Romanian Foreign Minister Bogdan Aurescu said was unacceptable.
Hungarian Foreign Minister Peter Szijjarto said he expected "more modesty from a foreign minister whose prime minister is currently facing trial", referring to an ongoing corruption case.
The EU has been strongly criticised for its failure to co-ordinate a response to the migrant crisis.
On Monday, foreign ministers from Hungary, Poland, Slovakia and the Czech Republic are due to discuss the crisis.
EU Interior ministers will hold an emergency meeting on Tuesday in another attempt to agree on relocating migrants with binding quotas for each state.
The next day, EU leaders will hold an extraordinary summit on migration.
Migrant crisis in Europe: Key recent dates
- 13 July: Hungary starts building razor-wire fence on border with Serbia
- 25 Aug: Germany says all Syrian refugees can apply for asylum there, regardless of which EU country they first entered
- 2 Sept: Image of body of three-year-old Syrian Alan Kurdi, washed ashore in Turkey, moves public opinion worldwide
- 12 Sept: Record 4,330 migrants cross into Hungary
- 13 Sept: In a switch of policy, Germany introduces border controls with Austria - other EU nations later impose their own controls
- 15 Sept: Hungary enforces tough laws on migrants crossing its border fence, prompting thousands to turn to Croatian route
- 18 Sept: Croatia transports migrants over its border into Hungary, which ships them on to Austria
- 19 Sept: Thousands cross into Austria, which prepares to send many on to Germany
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.
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