The death toll from floods and landslides following cyclonic rains in the Indonesian island of Sumatra has risen to 303, the head of the country's disaster mitigation agency said on Saturday, up from a previous figure of 174 dead.

Large parts of Indonesia, Malaysia and Thailand have been stricken by cyclone-fuelled torrential rain for a week, with a rare tropical storm forming in the Malacca Strait.

At least 279 people are still missing even as about 80,000 people have been evacuated and hundreds are still stranded in three provinces across Sumatra island, Indonesia's westernmost area, head of the agency Suharyanto told journalists.

Responders have used helicopters to deliver aid and for logistics in the northern part of the island, which was the hardest hit with roads cut off and communications infrastructure destroyed by landslides.

"We are trying to open the route from North Tapanuli to Sibolga (in North Sumatra province), which is the most severely cut off for a third day," he said.

He added that rescue forces were trying to break through a road blockage caused by a landslide, and that people were trapped on a stretch of road and in need of supplies. The military presence will be enhanced on Sunday to help with relief efforts, he said.

There were attempts by those affected by the rain to ransack supplies in the Central Tapanuli area, which was badly affected, he further added.

Across the Malacca Strait in Thailand, the death toll from floods in the southern part of the country has risen to 162, government spokesperson Siripong Angkasakulkiat said on Saturday, up from the previous toll of 145.