A person screamed for sleeping residents to flee their homes as fire raced through a block of Invercargill flats early today.

Despite the desperate effort to save everyone a man died inside his Tramway Rd unit as flames engulfed the block of five flats just after 1am.

Police are now trying to identify the man and locate his next-of-kin.

A neighbour who did not want to be identified said he was woken by the sound of a raised voice desperately trying to wake up residents inside the set of adjoining units.

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"I heard someone yelling 'get out, get out' and wondered what it was," he said.

Then he looked out to see the middle unit engulfed in flames and spreading quickly next door.

The Fire Service's Southland assistant area commander, Deane Chalmers, said when crews arrived at the scene two of the double-storey flats were engulfed by flames.

The fire had started to spread through the roof to adjoining flats.

"We had information that he was still in the building so the fire crew's main focus was to get that person," said Chalmers.

He said most of the occupants escaped unscathed, however the body of a man was later discovered in one of the units.

Searchers entered the flat and found a person who had not survived the fire.

Two units, including one where the man's body was found, had been completely destroyed by fire, he said.

The adjoining flats suffered significant fire and smoke damage.

The neighbour said the flats were mainly occupied by couples and single people in their 20s to 50s.

Residents who escaped the blaze with little more than the clothes they were wearing to bed were being looked after by Victim Support.

A police spokeswoman said the cause of the blaze was not known at this stage.

The scene was cordoned off with fire investigators now working out how the fatal fire started.