Hundreds of troops, engineers and social workers have converged on the desert town of Ghardaia to help with relief operations after a flash flood there killed 33, Algeria's interior ministry said.

Torrential rains caused rivers in this usually arid region bordering the Sahara to overflow in the past two days, destroying hundreds of houses in the historic town and several neighbouring oases.

Late Friday, the interior ministry said troops were securing the zone to avoid looting and helping to search for more potential victims.

Thousands of animals also drowned, and crops and food reserves were heavily depleted, it said. The military brought in two mobile bakeries to cook emergency bread rations and is helping rehouse survivors in public buildings.

About 360 tonnes of food, 1,000 tents and 200,000 blankets were being distributed to survivors, the official APS news agency said.

Some residents said rescue operations were conducted too slowly, and a local journalist said about 1,000 protesters gathered in Ghardaia's centre on Friday to demand more aid and additional equipment to search for victims.

Jamel Kechmad said speculation was rampant in the town that the death toll was much higher than officially reported. He spoke to AP by telephone.

Ghardaia, with a population of 100,000, is located in a long and narrow valley known as the M'zab about 600 km south of the capital, Algiers. The M'zab is listed by UNESCO as a World Heritage site, and lies on the edge of the Sahara, the world's largest desert.

The region is home to the Mozabite people, who practise a unique form of dissident Islam. The area saw violent clashes this year between locals who usually belong to the Berber ethnic minority and mainstream Sunni Muslims.

Both groups took part in Friday's protest, though they did not mingle, Kechmad said. Police officers and soldiers stood guard.

Thunderstorms can occasionally bring massive rains to the region, with engorged rivers then causing serious damage.

Residents said during this week's rains, a local river rose by 8 metres, triggering the floods. Mud flowed chest-deep in some streets and carried cars away, video footage showed.