LAGOS - At least four people were killed after a 21-storey building under construction collapsed Monday afternoon in the city of Lagos, Nigeria's economic hub, an emergency management official said.

Ibrahim Farinloye, the coordinator of the National Emergency Management Agency (NEMA) in the Lagos state, told Xinhua on the phone the 21-storey building under construction in the Ikoyi area of the Lagos city collapsed at about 3 p.m., local time, Monday, and so far four dead bodies have been recovered and four people been rescued alive.

Farinloye said the emergency phase of search and rescue is ongoing with additional assistance from the Nigerian army and air force, and more people are feared trapped.

He said what caused the collapse remains unclear and an investigation is underway.

Residents living in the area told Xinhua the building was under construction for more than one year, and more than 50 people, mostly construction workers, were trapped under the rubble.

Building collapses often occur in Nigeria as some property owners and developers do not adhere strictly to planning and building laws and regulations. In most cases, heavy loads are placed on the building and substandard materials are often used in construction.