At least six people have been killed after shells hit a cultural centre and a trolleybus in the Ukrainian city of Donetsk.

People had been queuing for aid outside the building in the centre of the city when two artillery shells struck, witnesses said.

Sky's Katie Stallard, reporting from the scene in Donetsk, said: "We have seen five bodies at this location.

"We saw the bodies of two middle-aged women and two men on the ground. Another man was killed in a car a very short distance away.

"We are told he was trying to manoeuvre the car away as the shell came in, but he was killed as he sat in the driver's seat."

Pictures from outside the cultural centre showed vehicles punctured by shrapnel and with their windows blown out. The tops of nearby trees had been blown off by the force of the blast.

The victims appeared to be dressed in civilian clothing.

The sixth victim was seen by a Reuters cameraman near the damaged trolleybus elsewhere in the city.

It was not immediately clear if the shells were fired by government troops or pro-Russian separatists.

A battle was also taking place around Debaltseve, a key government-held town of 25,000 people that straddles a railway connecting the two rebel centres of Luhansk and Donetsk.

The separatists claimed to have almost encircled the town, telling Ukrainian troops through Russian state TV: "Surrender and you will live".

The past week has seen some of the heaviest fighting in eastern Ukraine since a ceasefire was agreed five months ago.

Conditions in Donetsk have been deteriorating as the violence has ramped up, with hundreds of people taking shelter in an arts centre too afraid to let their children play outside or go to school.

More than 5,000 have died since the pro-Russian separatists declared their own "People's Republics" in the region last April.

The rebels said peace talks due to take place in the Belarussian capital, Minsk, on Friday, had been called off after Kiev failed to send any representatives.

Meanwhile, NATO has announced it will deploy small units in Estonia, Latvia, Lithuania, Poland, Bulgaria and Romania to help co-ordinate military exercises and set up command and control in response to Russia's action in Ukraine.

The forward units are expected to comprise only a few dozen troops, with Norway, Germany and the Netherlands expected to contribute.

Moscow has accused the European Union of adopting a "one-side reading" of the conflict and warned against further sanctions.

And Russia's military chief said on Friday that the country's strong nuclear arsenal would continue to ensure its military superiority over the West.

Armed forces chief General Valery Gerasimov said the military would receive more than 50 new intercontinental ballistic missiles this year.