ASUNCION, Aug 2: Investigators in Paraguay searched for evidence on Monday that security guards might have trapped shoppers in a burning supermarket by locking the doors to stop looting , while the death toll in the country's worst tragedy in decades rose to nearly 300.

President Nicanor Duarte Frutos called for a rapid investigation into Sunday's blaze "so those responsible are punished with the full force of the law."

"There are several witnesses who saw how they shut the doors to the supermarket and we also confirmed that the emergency exit was welded," Paraguay's Police Chief Humberto Nunez told Reuters.

Three owners of the Ycua Bolanos supermarket and three security guards are in police custody, but main shareholder Juan Pio Paiva said no orders were given to lock the doors.

"The company has insurance against vandalism," he told a news conference. "It does not make sense in a fire of this magnitude for security guards to close the doors and stay inside."

The fire swept through the busy supermarket on the outskirts of the capital Asuncion at midday on Sunday when it was packed with shoppers. Officials said the blaze was caused by a gas explosion near the food court.

"As we ran to the exit, I looked back and saw a ball of fire and the roof fall on several people," said survivor Liz Torres, who was not classified as being in "serious condition" but whose face, lips and hands were completely blackened. -Reuters