At least 500 migrants are believed to be on board a freighter adrift south of the island of Crete, the Greek coastguard said this morning.
"We estimate that there are between 500 and 700 people on board," a coastguard spokeswoman said.
The Kiribati-flagged freighter Baris has been adrift owing to mechanical failure 56km southeast of Crete since early this morning.
Strong winds are blowing in the area and the weather is deteriorating, the spokeswoman said.
"There are currently five vessels in the area, including a Greek navy vessel. The migrants will either be picked up or the freighter will be towed to the nearest port," she said.
Greece is one of the main points of entry into the European Union for people fleeing war-torn and impoverished countries in Africa, the Middle East and the Indian subcontinent.
Many of the recent arrivals are escaping civil war in Syria.
People traffickers are increasingly using Greece's Aegean islands to smuggle migrants into Europe after the building of a fence and tighter controls along its land border with Turkey.