Rescuers carry a child rescued from the scene of a building collapse in Lagos, Nigeria, on March 13, 2019. (Xinhua/NAN)

UNITED NATIONS, March 14 (Xinhua) -- UN Secretary-General Antonio Guterres was "shocked and saddened" by the fatal collapse of a building in Nigeria, his spokesperson said Thursday.

"The secretary-general will be writing to the government of Nigeria to express his condolences," Stephane Dujarric, the spokesperson, said at a regular press briefing.

On Wednesday, at least 12 pupils have been confirmed dead following the collapse of the three-storey building which housed a primary school in Lagos in southwestern Nigeria, according to a hospital source.

Governor of Lagos State Akinwunmi Ambode said a thorough investigation will be launched into the incident.