Five women were among the dead in the powerful blast leading to two adjoining buildings caving in due to its impact in the textile city early on Sunday, police said.

Surat police commissioner V.K. Gupta said it was believed that several cooking gas cylinders had exploded.

The early morning explosion in the city of Surat, 250 km from Gujarat's commercial capital Ahmedabad, was so powerful that two adjoining buildings also collapsed, he said.

The gas was being used to make holders for cutting and polishing diamonds at a small factory, he said.

Gupta said the workers were asleep in the factory at the time of the blast.

"We believe there may be 15 or 20 more workers trapped inside, though we cannot be sure," he said.

The injured were admitted to three hospitals in the city and about 20 of them were in a serious condition, police said.

An investigation was under way to determine the exact cause of the accident.

Another possibility being probed was whether sub-standard material was used in the construction of the buildings that could have triggered the collapse after the blast, police said.

The impact of the explosion in mini Saurashtra Varachha area was so strong that two adjoining buildings, having commercial establishments, also came crashing down, official sources said.

The entire area has been cordoned off and rescue and relief operations were being conducted on a war footing. Senior district and police official are supervising the operations.

"Fifty persons are still trapped under the debris, we have rescued 35 persons," Surat Range Commissioner of Police VK Gupta said.

The injured have been admitted to three different hospitals and condition of three was critical, police said adding 19 persons were discharged after treatment.