A total of 18 people have died as a result of a period of extreme cold weather in New York City, its mayor has said.

Since late January, the city has endured a cold snap, including 13 days of temperatures of 0 °C (32F) or below – one of the longest stretches of sub-zero weather New York has seen in six decades.

Over the weekend, another person "lost their life on the streets of our city," Zohran Mamdani said on Monday, adding that "each life lost is a tragedy, and we will continue to hold their families in our thoughts".

While temperatures are set to rise this week, they remain below average, with Mamdani telling residents to "stay safe, stay indoors... [and] keep looking out for one another".

The mayor added that since 19 January, when a Code Blue emergency was announced - which relaxed intake policies for homeless shelters - about 1,400 placements had been made into shelters.

An additional 64 hotel rooms had been added to the city's shelter capacity, with at least another 150 outreach workers on the streets, said Mamdani.

On 27 January, Mamdani said at least 10 of the people who had died were found outdoors. The circumstances of the other deaths are not known.

"We have been working hard to keep New Yorkers safe, and we will continue to do so. Because it is not forecast to be above 32 degrees [Fahrenheit] until tomorrow, and 35 degrees is hardly balmy weather," he said.

The US National Weather Service (NWS) said temperatures would increase to about 0 °C for the most part of the week, following persistent freezing temperatures on Monday. It added that highs were normally around 4 °C at this time of year.

An Arctic airmass brought hazardous sub-zero "wind chills" - the temperature it feels like rather than the temperature it is - to the region over the weekend, bringing a risk of hypothermia and frostbite, the forecaster said.

A 13-day streak of temperatures being at 0 °C or below -one of the longest streaks for New York City since 1963 - ended on Friday after temperatures briefly tipped above 0 °C.

NYC's Emergency Management agency said that the following days of extreme cold and snow pose serious safety risks across the city.

It warned that melting snow and ice could fall from buildings without warning and that streets and pavements could refreeze overnight.