A 21-storey building under construction in Nigeria's main city, Lagos, has collapsed.

Eyewitnesses told the BBC that several people are believed to be trapped inside the building, including construction workers and contractors.

The Lagos State Emergency Management Agency says it has activated its emergency response plan to deal with the situation.

Meanwhile, security agencies are struggling to cordon off the area as crowds gather around the site.

The cause of the collapse of the building is still unclear.