Riga, Nov 23 (IANS): The death toll in the roof collapse at a supermarket in the Latvian capital Riga Thursday has risen to 51, police said.

The number was expected to rise further as some 30 people, or maybe more, are still feared trapped under the rubble, Xinhua cited police as saying.

The rescuers were still trying hard, listening for voices of people trapped, but "unfortunately the only voices out there are of firefighters working", said state police spokesman Toms Sadovskis.

The supermarket, built in 2011, belongs to the Lithuanian-owned Maxima chain, the second largest retailer in the country.

Rescue work is proceeding slowly as both the rubble and remaining sections of roof are at risk of further collapse.

The reason for the collapse is currently not known, but reports said a garden was being built on the store's sizeable roof, which might have violated construction regulations and triggered the collapse.

The government has announced three days of mourning from Saturday.