More than 20 people are now known to have died as icy temperatures continue to grip much of Europe.
Ten people died of cold in Poland on Sunday. The bodies of three migrants, two Iraqi and one Somali, were found near the Turkish-Bulgarian border.
Deaths have been reported in Italy, the Czech Republic, Russia and Ukraine.
Dozens of flights have been halted. In Turkey, the Bosphorus is closed to shipping after a snowstorm. Even Greece's islands are covered in snow.
In Serbia, all river transport is suspended on the Danube.
Your pictures: Icy weather causes disruption across Europe
The Medecins Sans Frontieres aid organisation said it was "very concerned about the thousands of vulnerable people across the continent in danger and stuck in undignified conditions".
It said "of particular concern are the 2,000 people living in informal settlements in Belgrade where temperatures are currently reaching -20C (-4F)". It added that the majority of them were young people from Afghanistan, Pakistan, Iraq and Syria.
In Russia, temperatures in some regions have plunged below -40C.
Normally milder Greece has witnessed temperatures of -15C in the north, where an Afghan migrant died of cold last week and roads were closed.
In Athens, the temperature failed to rise above 0C over the weekend and many of the islands were covered in snow.
Some of the Greek islands are home to thousands of migrants and many are being moved to temporary housing and heated tents.
Homeless hostels in Italy are opening their doors day and night as the weather has been blamed for the deaths of seven people, five of them living out in the open.
Airports in Sicily, Bari and Brindisi closed over the weekend. Rome, too, saw freezing temperatures.
In Russia, Moscow experienced its coldest Orthodox Christmas in 120 years on 7 January.
Dozens of flights have been cancelled or delayed in the capital.
Schools in several regions of the country are closed on Monday, however officials say milder temperatures are expected in the coming days.
Prague had its coldest night so far this winter, Czech officials say, with six deaths reported, most of them homeless people.
With the 10 deaths confirmed in Poland on Sunday, officials say country's death toll from winter weather has risen to 65 since 1 November.
The cities of Warsaw and Krakow are on Monday providing commuters with free public transport as the authorities try to tackle high smog levels linked to the cold weather.