Five migrants were killed on Tuesday when their boat sank in the Aegean Sea while trying to cross from Turkey's western coast to EU member Greece, reports said.

Sixteen more people were rescued by air and by sea in a search and rescue operation by the Turkish coastguard, the Dogan news agency said.

Turkey will boycott talks seeking to find peace for Syria planned later this week in Geneva if the Syrian Kurdish group the Democratic Union Party (PYD) is invited, the Turkish foreign minister said on Tuesday.

If the PYD is invited "then of course we will boycott" the talks in Geneva, Foreign Minister Mevlut Cavusoglu said in a live interview on the private NTV television, adding that the PYD is a "terrorist organization" which cannot represent Syrian people.

Turkish forces have detained 23 suspected Islamic State (IS) jihadists along with 21 children who were trying to illegally cross over from Syria, the army said Sunday.

The suspects, whose nationalities were not disclosed, were captured on Saturday as they tried enter the Elbeyli district of Turkey's southern Kilis province.

U.S. Vice President Joe Biden on Friday strongly criticized Turkey for failing to set the right "example" on freedom of expression, slamming the imprisonment of journalists and investigation of academics who criticized government policy.

In a rebuke of rare vehemence for Washington's key NATO ally, Biden said Turkey's most recent record in freedom of expression under President Recep Tayyip Erdogan is "not the kind of example that needs to be set".

Five children were wounded on Friday when suspected militants hurled a hand-made explosive device into a schoolyard in majority-Kurdish southeast Turkey, security sources said, blaming the attack on the Kurdistan Workers Party (PKK).

The blast ripped through the yard of a secondary school in the Baglar district of Diyarbakir province where students had gathered during a classroom break, a security source told AFP.

Turkish President Recep Tayyip Erdogan on Friday expressed alarm over reports of a build-up of Russian troops in northern Syria near the Turkish border, saying such movements would not be tolerated.

"We have said this from the beginning: we won't tolerate such formations (in northern Syria) along the area stretching from the Iraqi border up to the Mediterranean," Erdogan told reporters after Friday prayers in Istanbul.

German Chancellor Angela Merkel said Friday she had obtained a pledge from Turkey to "do everything" to cut the record number of migrants arriving in Europe.

After talks with Prime Minister Ahmet Davutoglu in Berlin, Merkel told reporters they had signed a joint communique under which "the Turkish government will do everything to reduce the number of refugees" crossing into the European Union.

At least 12 migrants were killed and several more went missing Thursday when their boat sank while trying to cross the Aegean Sea from Turkey to EU member Greece, Turkish media reports said.

The boat, carrying some 50 migrants, struck trouble after leaving the western Turkish resort of Foca in the Izmir region for the Greek island of Lesbos.

Turkish Prime Minister Ahmet Davutoglu on Thursday urged "concrete steps" from the European Union to help Turkey tackle the refugee crisis, a day before he holds critical talks with German Chancellor Angela Merkel.

Speaking at the annual Davos summit of economic and business leaders, Davutoglu said he wouldn't even ask Merkel about the 3 billion euros in financial aid the EU promised last year to grant to Turkey in return for stemming the flow of migrants into the bloc.

U.S. Defense Secretary Ashton Carter said Turkey "can do more" in the fight against the Islamic State group, particularly by tightening its border to stop the flow of resources and foreign fighters.

"Turkey occupies a key position in the coalition -- it is hosting aircraft and making other contributions," Carter told reporters in Paris, where he has been meeting defense ministers from several countries involved in the anti-IS coalition.