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Solothurn - Six people, including children, died when a fire broke out in an apartment building northwestern Switzerland, apparently due to "incorrect use of articles for smokers," police said Monday.

One person was arrested following the deadly blaze, which happened in the early hours of Monday in the town of Solothurn, north of the capital Bern.

"There were more than 20 people in the building. Most were evacuated by the fire brigade, but for six people, including children, the help came too late," Solothurn police said in a statement.

"They died on the spot."

Four of the survivors were hospitalised, some with serious injuries.

Police said an initial investigation in the three-storey block indicated the fire started on the lower level, spilling smoke upwards, and was likely caused by the faulty use of smokers' items, but they did not specify what material they were talking about. 

The identity and connection of the person detained were not given, nor were the identities of those killed.

But neighbours were quoted by several media outlets suggesting that asylum seekers were living in the building. A local cafe owner told AFP he believed they were mainly Eritreans.

"A resident noticed smoke at 2:10 am (0110 GMT) in the stairwell and sounded the alarm," the police statement said, adding that dozens of firefighters, along with police, ambulances and other rescue services were immediately dispatched to the building.

- 'Impossible to escape' -
Solothurn city councillor Kurt Fluri told the 20Minutes online news site that he was deeply saddened by the tragedy. 

He said the fire actually appeared to have been quite small.

"It was the smoke that was the problem. It was impossible to escape from it," he said, adding that some people "appear to have jumped out of the window".

Another witness, a man who had been evacuated from a neighbouring building, also told 20Minutes he had heard that some people had jumped to escape the smoke and flames.

"They panicked," he said.

Another neighbour, 62-year-old Franco Pedone, told the Blick newspaper's online edition that he had been jolted awake.

"I suddenly heard women shouting in the night. They were screaming like crazy," Pedone said. 

And yet another neighbour told 20Minutes that he had heard screams of people stuck in the building and had seen a child being resuscitated by emergency workers.

"That was the worst," he said.