Nigeria's main city of Lagos was shaken for a third straight day Monday by fighting between ethnic gangs which has left at least 21 dead in a working-class district.

Many fear the toll from the fighting which erupted on Saturday in the Idi-Araba area of the Mushin district will be significantly higher. One newspaper on Monday put the toll at 40 dead while another said 70. Those figures could not be independently confirmed.

Scores of people have fled their homes and dozens of buildings have been set on fire.

Smoke could be seen drifting from the area early Monday, mingling in the air with the seasonal harmattan dust, blown down every year from the Sahara desert far to the north.

Ayo Olagunju, spokesman for the Lagos University Teaching Hospital (LUTH), told AFP by telephone that fighting had resumed early on Monday after an overnight lull, a pattern seen overnight Saturday.

"They have started fighting again. The situation is serious. They are burning houses and fighting," he said.

The hospital is near the worst area of the fighting.

Olagunju was unable to provide an estimate of the latest death toll. But hospital workers and AFP reporters said on Sunday that at least 21 people had been killed.

An AFP reporter who visited the area on Sunday saw six bodies being brought into the hospital, which was just officially reopening Monday after a nine-week health sector strike.

Olagunju said "hundreds" of terrified people had fled into the hospital grounds to seek shelter.

Police had left the area, radio reports said.

The fighting erupted on Saturday between gang members of the ethnic Yoruba and Hausa communities in the district when a young Christian ethnic Yoruba boy was caught defecating near a mosque.

What was considered the desecration of the mosque enraged members of the local Muslim Hausa trader community, which worships at the mosque, and an ethnic clash erupted.

Speaking late Sunday, Lagos State Governor Bola Tinubu, already struggling to cope with the aftermath of last week's devastating munitions store explosion which left more than 1,000 dead, blamed the violence on "area boys", the notorious Lagos street gangs.

"Hausas and Yorubas in Idi Araba have been living peacefully together for years," he said.