A four-story under-construction building collapsed at Ullawas village in Sector 61 here on Thursday morning.

The building collapsed around 5 a.m. and a call was received at the Fire Service Department 20 minutes later, said Assistant Divisional Fire Officer I.S. Kashyap. He said that the building seemed to be an unauthorized construction and tragedy happened as extra floors were added to an already weak structure.

More 500 rescue personnel, including National Disaster Response Force, were pressed into the rescue operation.

"Two backhoes were being used to remove the debris and pull out those trapped inside. It is a congested area causing difficulty in carrying out the rescue operation at a faster pace. The locals said that 5-6 people were staying in the building. All are feared trapped," said Mr. Kashap, adding that the cutters were also being used to cut through the debris and pulled out those trapped.

"The building belongs to a person named Dayaram. Around 20 people, mostly security guards employed and deployed in the nearby sectors were living there on rent."

Gurugram Police spokesperson Subhash Boken said the fourth floor was added to the building on Wednesday and its owner belonged to the village.