Palestinians carry the flag-draped bodies of people who died a day earlier, when fire broke out in an apartment, in Gaza strip, on Friday. AFP

Mourners flooded the streets of Gaza on Friday as they carried the shrouded bodies of 21 people, including at least eight children, killed in a fire in an apartment block where residents had planned to celebrate the homecoming of a relative.

Twenty-one victims of the fire that tore through a top-floor apartment in the Gaza Strip during a birthday party were members of the same family, two of their relatives said.

Palestinian President Mahmoud Abbas called it a national tragedy and declared Friday a day of mourning. Several Arab states, the United Nations and the European Union expressed their condolences for the bereaved families.

At least eight children were among the dead in the blaze at Jabalia Palestinian refugee camp on Thursday night, said the head of the camp's Hospital, Saleh Abu Lai.

It took firefighters more than an hour to get control of the massive flames that burst through the top floor of a residential building in Jabalia, one of eight refugee camps in the Gaza Strip, where 2.3 million people live in one of the most densely populated areas on earth.

Details about how or why the fire broke remain unclear as there were no survivors, Abu Ahmad Abu Rayya, the head of the clan, said.

Shrouded in Palestinian flags, the dead were carried through crowds of mourners to Beit Lahia cemetery where they were buried.

Officials in Hamas-run Gaza have said Thursday night's blaze in a three-storey residential building in the Jabaliya refugee camp was apparently fuelled by stored gasoline. They said it was not clear how the gasoline ignited, and that an investigation is underway.

"We do not have the means to put out large fires, especially in narrow places and high buildings," a senior civil security official told AFP.

It was one of the deadliest incidents in Gaza in recent years outside the violence stemming from the Israeli-Palestinian conflict.

The blaze destroyed the top-floor apartment in the building, home to the Abu Raya family.

Mohammed Abu Raya, a family spokesman, told The Associated Press that the extended family had gathered for twin celebrations — the birthday of one of the children and the return of one of the adults from a trip to Egypt.

Abu Raya spoke at the Indonesian Hospital in northern Gaza, where the bodies had been taken and where sobbing relatives were waiting for funeral processions to begin.

Abu Raya challenged assertions that stored gasoline fuelled the blaze, saying furniture made from flammable materials was more likely to have accelerated the flames.

"The disaster was that no one came out alive to tell us the truth of things,” he said. "I do not think that it was stored gasoline.”

Those killed were from three generations — a couple, their five sons and one daughter, two daughters-in-law and 11 grandchildren, according to Abu Raya and Mohammed Jadallah who had married into the Abu Raya family.

Agencies