By Nneka Chile

LAGOS (Reuters) – A luxury residential high-rise under construction in Nigeria’s commercial capital Lagos collapsed on Monday, trapping several workers under a pile of concrete rubble, witnesses said.

Two workers at the site in the affluent neighborhood of Ikoyi, where many blocks of flats are under construction, told Reuters that possibly 100 people were at work when the building came crashing down.

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

There were heaps of rubble and twisted metal where the 20-story building once stood, as several workers looked on. One man wailed, saying his relative was among those trapped.

It was not immediately clear what caused the collapse.

The building was part of three towers being built by private developer Fourscore Homes. In a brochure for potential clients, the company promises to offer “a stress-free lifestyle, complete with a hotel flair.” The cheapest unit was selling for $1.2 million.

Calls to the numbers listed for Fourscore Homes and the main building contractor did not ring through.

The Lagos State Emergency Management Agency said it had activated its emergency response plan. “All first responders are at the scene while heavy duty equipment and life detection equipment have been dispatched,” the agency said in a statement.

(Additional reporting by Tife Owolabi in Yenagoa, writing by MacDonald Dzirutwe, editing by Nick Macfie and Mark Heinrich)