Sub-freezing temperatures across swathes of Europe have claimed over 60 victims in recent days -- notably in Poland, Romania and the Balkans -- with migrants and the homeless among the most vulnerable, officials said on Tuesday.

Six people died in Poland and seven more in the Balkans over a 24-hour period as temperatures plunged across Europe, while Romania announced six deaths in recent days.

Authorities in Warsaw said the toll of hypothermia deaths in the country since November 1 had now risen to 71.

Polish police urged people to help those most vulnerable, especially the homeless, as the mercury sank to below -20 degrees Celsius (minus four degrees Fahrenheit) in some regions.

"Another six people died due to exposure over the last 24 hours," the centre for national security (RCB) said on Tuesday in a statement.

Local authorities have also issued smog alerts across Poland, urging children and the elderly to remain indoors as coal and waste-fired home furnaces drive up air pollution to the highest levels recorded in years.

Last winter in Poland was unusually mild but still claimed 77 lives in the nation of 38 million, compared to 78 in 2013-2014 and 177 in 2012-2013.

- Miraculous rescue -

In the Balkans, a father and son were amongst at least seven people reported to have died in the last 24 hours, local media said -- three each in Serbia and Macedonia, and one in Albania.

In southern Serbia, where temperatures have fallen below -20 C and a state of emergency has been declared, an 88-year-old father and his 64-year-old son died of the cold in the village of Duga Poljana.

But in the east of the country there was rare good news as national television station RTS reported the miraculous survival of a man in the village of Smoljinac who survived for two days in temperatures reaching minus 20 Celsius after falling down a well.

Finally rescued by a neighbour, he was taken to hospital.

In Slovakia, six people have succumbed to the cold weather in recent days, according the country's Central Emergency Services told AFP Tuesday.

The current deep freeze sweeping Europe has also killed six people in Romania in recent days, according to first official figures released by the health ministry in Bucharest.

This raises the total number of deaths across the continent to over 60 in just a few days, with many of the victims being migrants or homeless people.

- No heating -

In Greece, which has more than 60,000 mainly Syrian refugees on its territory, many migrants have been moved to prefabricated houses and heated tents.

It has also sent a tank landing craft to Lesbos to pick up refugees and migrants currently living in miserable conditions in the Moria refugee camp, navy spokesman Spyridon Pollatos told AFP.

The craft "should arrive on Wednesday and can house 500 people", Pollatos said.

More than 2,500 people, "including children, women and disabled people" are living "in tents, without either hot water or heating" at the Moria camp, said Apostolos Veizis of Doctors Without Borders.

Since Friday, Greece has been hit by an unusually cold snap and heavy snowfall.

Meanwhile, traffic on the Danube between Romania and Bulgaria was halted indefinitely on Tuesday.

Schools remained closed in large parts of Romania, with more snow storms forecast for Tuesday evening.

Heavy snowstorms also reached Turkey over the weekend, paralysing its biggest city Istanbul where almost 65 centimetres (25 inches) of snow fell, forcing hundreds of flights to be cancelled on Saturday.

The Russian capital recorded its coldest Orthodox Christmas Night for 120 years at the weekend, according to media reports.