A high-rise building under construction in Nigeria's commercial capital Lagos collapsed on Monday, killing at least six people and trapping an unknown number under a pile of concrete rubble, according to the state emergency agency's chief.

The building in the affluent Ikoyi area of the city collapsed at about 2:45 p.m. local time. It remains unclear what caused the collapse. The authorities have pledged to make the results of their investigation public. 

Two workers at the site told Reuters that about 100 people could have been at work when the building came crashing down. Many blocks of flats are under construction in the area.

Building collapses are frequent in Nigeria, Africa's most populous country, where regulations are poorly enforced and construction materials are often substandard. 

In 2014, 116 people died in the collapse of a building in the compound of a Lagos church. And in 2019, 10 people died in a school collapse, also in Lagos.

This particular building was being developed by Fourscore Homes Limited, which has many current projects in other areas of Nigeria and across the globe.