Istanbul/London: Twenty-seven migrants, 11 of them children, drowned off Turkey's Aegean coast as they tried to reach a Greek island, the Turkish coast guard said.
Four migrants were rescued and a search operation was underway for nine remaining passengers.
One migrant was rescued by a fisherman and three more were rescued by the coast guard, which said it had deployed boats and helicopters to search for more passengers.
The boat sank in the Aegean Sea near the Edremit area of the northwestern province Balikesir.
Separately, the private news agency Dogan said 11 migrants died and three were rescued when another boat sank further south, off the coast of Dikili in the province of Izmir.
More than 900,000 people fleeing Syria, Afghanistan and other war-torn or impoverished countries arrived in Greece from Turkey last year, often risking their lives in the short but perilous sea crossing in overloaded boats. Hundreds have died making the attempt.
Meanwhile, a spokesman for Prime Minister David Cameron said on Monday that thousands of refugees could flock to Britain from France if voters decide to leave the European Union.
In what critics said was the start of a "campaign of fear" to try to keep voters in the European Union, the spokesman said leaving the bloc could harm an agreement with France which allows British border guards to make immigration checks there.
At a news conference, Cameron backed up the message by saying the agreement, which puts Britain's border inside France, such as at the northern port town of Calais, was a good deal for Britain and one he would fight to keep.
"If... we can stay in a reformed Europe, you know what you get," he said.
"You know that the borders stay in Calais, you know that we have a seat determining the rules when it comes to the future of Europe, you know we have that vital information whether it's about terrorists or criminals travelling around Europe because we are part of those organisations."
He said there were "a lot of opposition politicians" in France who would like to rip up that agreement, which would open the gates to thousands of refugees now living in Calais' so-called "Jungle" camp in the hope of crossing to England.
"I don't want to give people an excuse to do that."
Earlier his spokesman said thousands of refugees could cross the Channel into England "overnight" if Britain voted to leave the European Union at a referendum, which could take place as early as in June.
A Dutch government minister said on Monday that the Netherlands has stepped up the monitoring of traffic crossing its frontiers.
The move follows a decision last September to deploy mobile border guard units along roads and railways lines to intercept migrants and separate them from asylum seekers.
"Those measures helped, but we were still seeing a rising number of smugglers being arrested, and you are seeing more people coming from safe countries, who aren't eligible for asylum," state secretary Klaas Dijkhoff told reporters.
The Netherlands received 200,000 immigrants last year, which fuelled a backlash against immigration in a country that was once known for the generosity of its open-borders policy. In the 1960s, a period of high growth, the country opened its borders to migrant workers from Turkey and Morocco.
By slowing traffic as it crossed the border, border police would be better able to pick out vehicles that needed closer attention, Dijkhoff said. Last year, Dutch border police arrested 330 people on suspicion of people smuggling, 200 of them during mobile border checks.
The measures would mean that arrivals from safe countries and those who had made asylum applications in several countries could be told there was no point in going further, he said.
Safe countries are designated under European rules, meaning migrants from those countries cannot claim asylum on the grounds of war or the risk of oppression or torture.
The aim was not to reimpose border controls as Europe seeks a solution to the crisis, Dijkhoff said. "We want to reach a deal with Greece and Turkey to close the borders, but until that happens we will have to take our own measures."
In Athens, Greece said on Monday it would have migrant registration centres running by next week, under pressure from EU partners threatening to sideline it from passport-free travel unless it does more to contain refugees.
Defence Minister Panos Kammenos said Greece would meet a mid-month deadline to complete five "hot spot" centres to register refugees on the Greek islands which are closest in proximity to Turkey, and two relocation centres on the mainland.
"The Defence Ministry has undertaken a pledge to complete the work for the centres by the 15th (of February)," Kammenos told reporters.
Scuffles broke out on Friday and Saturday on the Greek island of Kos between police and a small number of residents protesting at the construction of a registration centre for migrants. There were also protests reported close to the northern city of Thesaloniki on Sunday, and rival protests in Pireaus close to Athens on Monday.
Residents were worried that the arrangement could be for the long-term, said Kos mayor Georgios Kyritsis. "One wonders if the creation of such facilities may be an incentive for people smugglers," he told Greek Skai TV.
In Anakara, Turkish Prime Minister Ahmet Davutoglu said that Turkey will admit the almost 30,000 people fleeing war-torn Syria who have amassed at the border "when necessary," adding that Russia's air assault should not be tolerated with the idea that Turkey will accept the refugees.
The latest developments in Syria are an attempt to pressure Turkey and Europe on the migration issue, Davutoglu said at a joint news conference with German Chancellor Angela Merkel.
Davutoglu also said that Turkey would inform Brussels next week on the initial projects it plans after receiving 3 billion euros in funds from the EU aimed at curbing record flows of migrants to Europe via Turkey, where 2.2 million Syrians are already sheltering.
Merkel said that donors should stop talking and start building projects to help refugees in Turkey, following European promises of billions of dollars in aid last week.
"We need a visible first project. It doesn't help a child from Syria that is a refugee here, or a Turkish class that has to share its room with Syrian refugees to say we have pledged 3 billion. They want to see a school in the city and fast," Merkel said.
"We need to work on this. We need to make sure there are not too many bureaucratic hurdles. Rather the refugees have to see the benefits quickly and without bureaucracy."
Merkel also said she was "appalled" and "shocked" by the suffering in the Syrian city of Aleppo, which she blamed on bomb attacks originating primarily from the Russian side in support of the Syrian government.
She said the bombings have forced tens of thousands of civilians to flee and suggested Russia's actions might be in violation of a U.N. Security Council resolution that Moscow signed in December.
"We must take another look at Resolution 2254 from December 18, the resolution of the UN Security Council that was supported by Russia," Merkel said.
"In the resolution the Security Council demands that all sides stop attacks on civilians and civilian targets without delay, and in particular the use of indiscriminate weapons, such as bomb attacks from the air. It is very specific in the resolution."