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       ABUJA (Reuters) - At least six people have died in Nigeria's commercial capital of Lagos after the collapse of a high-rise building that was under construction, the state emergency services chief said on Tuesday.

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

       State official Olufemi Oke-Osanyintolu said a search and rescue effort had been launched for survivors of Monday's incident.

       "Currently all responders are on the ground as search and rescue is ongoing," Oke-Osanyintolu said, adding that the death toll now stood at six.

       Four people were rescued alive and three more treated for minor injuries at the scene, he said.

       Witnesses say up to 100 people are missing after the luxury residential structure crumbled, trapping workers under a pile of rubble.

       Rescue workers used excavators to sift rubble in the glare of floodlights powered by generators as heaps of shattered concrete and twisted metal engulfed the site where the building once stood, as more workers watched.

       President Muhammadu Buhari has called for rescue efforts to be stepped up as emergency services, including hospitals, swing into action.

       The collapsed building was part of three towers being built by private developer Fourscore Homes, which promised in a client brochure to provide "a stress-free lifestyle, complete with a hotel flair". The cheapest unit was selling for $1.2 million.

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

       (Reporting by Chijioke Ohuocha; Editing by Clarence Fernandez)