WARSAW - Sub-freezing temperatures across swathes of Europe have claimed more victims notably in Poland and Romania, with migrants and the homeless among the most vulnerable, officials said Tuesday.

Six people died in Poland over a 24-hour period as temperatures plunged across Europe , bringing the toll of hypothermia deaths in the country to 71 since November, authorities in Warsaw said, while Romania announced six more deaths in recent days.

Polish police urged people to help those most vulnerable, especially the homeless, as the mercury sank to below minus 20 degrees Celsius (minus 4 Fahrenheit) in some regions.

"Another six people died due to exposure over the last 24 hours," the centre for national security (RCB) said Tuesday in a statement, adding that "the number of hypothermia victims has reached 71," since November 1.

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.

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 there on Tuesday.

This raises the total number of deaths across the continent to at least 52 in recent days, with many of the victims being migrants or homeless people, with the Czech Republic. Greece, Italy and Macedonia among the other countries hit.

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

Traffic on the Danube between Romania and Bulgaria was halted indefinitely on Tuesday as temperatures plunged to below 30 degrees Celsius.

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.

Two people died of hypothermia in Moscow between December 31 and January 9, Russian news agencies reported Monday.