The Doctors Without Borders charity and environmental group Greenpeace have launched a joint operation to rescue migrants crossing the Aegean Sea from Turkey to Greece, they said Thursday.

The two organisations said that since November 28 they have been using three rigid hulled inflatable boats (RHIBs) based on the northern coast of Lesbos to help hundreds of people on the move, rescuing many of them from boats in obvious distress.

"Horrified by the unabated refugee crisis in the Mediterranean, we were compelled to act in whatever way we could," Alexandra Messare, Greenpeace Greece's programme director, said in a statement.

"We have brought our maritime experience to this collaboration ... in the hope of saving lives," she added.

Doctors Without Borders, which goes by its French acronym MSF, said that in addition to the boats it also had medical teams on standby at landing sites on the Greek islands to provide emergency care for people in critical condition, and sending those in need to hospital using three of its ambulances.

MSF said that in coming days it planned to add three more boats to help expand rescue capacity on other islands.

Marietta Provopolu, head of MSF's Greece division, said that "our activities in the Aegean are only a temporary solution to mitigate the suffering and loss of life we have seen in recent months."

"As the weather conditions worsen and the sea becomes even more dangerous, we know there will be new tragedies at sea," she added.

Nearly 895,000 people have crossed the Mediterranean to Europe so far this year, and more than 3,500 have died trying, according to the UN refugee agency.

The vast majority of those who have attempted the perilous journey are refugees fleeing conflict and persecution in Syria, Afghanistan and Iraq, and most of them have landed in Greece.

Despite worsening weather conditions, people are continuing to cross, with some 140,000 landing on Europe's shores in November.

MSF said its medical teams on Lesbos had carried out 6,154 consultations in the past two months, and had notices "rising numbers of people suffering from respiratory tract infections and hypothermia, linked to the increasingly rough seas and cold weather."

The two organisations urged the European Union to make it possible for the migrants and refugees to safely seek protection within the bloc.

To bring down the numbers attempting the dangerous sea voyage, they demanded that European countries create dignified and safe reception for migrants and refugees on the land border between Turkey and Greece.

"It is unacceptable that the main way to access international protection in Europe involves risking one's life at sea," said Stefano Argenziano, MSF's operations coordinator.