"We don't know where they came from," she said, adding that no further details were immediately available about the victims' nationalities.

A large search-and-rescue operation was underway, the spokeswoman said.

The reasons for the sinking were unclear: Winds on Saturday were a moderate 5 on the open-ended Beaufort scale.

"We cannot and should not tolerate losing people, losing children in the waters of the Aegean," Greece's migration minister, Dimitris Vitsas, said in a statement. "Clearly the solution is in finding measures to protect these people, in safe procedures and passages for refugees and migrants and in the relentless crackdown on human-trafficking rackets."

Newly arrived migrants join thousands of others in crowded state-run camps where frustrations are growing.

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In a joint statement issued this month, nine rights groups — including Human Rights Watch, Amnesty International and Oxfam — denounced the island camps as "open prisons" with "deplorable" conditions, and called on authorities to "immediately" transfer migrants to the Greek mainland.

Thousands have been moved to the Greek mainland in recent months, but authorities have avoided transferring them all amid fears that such a move would send a message to human traffickers that the road to Europe is effectively open.

The enforcement of the migration deal signed by Brussels and Ankara, Turkey, two years ago is expected to be discussed at a meeting of EU and Turkish leaders in Bulgaria on March 26.

New York Times