Romanian authorities said Wednesday they had rescued more than 150 migrants in the Black Sea, the fifth such incident since August amid rising fears that a new migrant route to Europe is opening up.

Authorities said 153 people including 53 children and 51 women, many from Iraq, had been on board the vessel when conditions turned choppy off Romania's eastern coast in the night from Tuesday to Wednesday.

"The sea had been very agitated, with waves of up three metres (9.8 feet). These people were in a very dangerous situation and risked either drowning or seeing their boat sink," Cristian Cicu, a spokesman for the Constanta coastguard, told AFP. 

AFP journalists in the Romanian port of Midia witnessed the arrival of the migrants who were then sent for medical examinations before being handed over to the immigration authorities.

EU member Romania is not part of the bloc's passport-free Schengen zone and until now has largely avoided the kind of influx of refugees and migrants seen elsewhere on the continent over the last few years.

But Bucharest fears that the Black Sea could become an alternative route for migrants seeking a new life in Europe as the dangerous journey across the Mediterranean becomes increasingly difficult, not least since Libya has sought to restrict charities carrying out rescue missions off its shores.

Until last month, only a handful of migrants had been seized in the Black Sea since the migrant crisis erupted in the summer of 2015.

But there has been a noticeable rise over the past weeks, with some 570 intercepted between August and September.