The flooding has deluged the homes and businesses of around a million people [AFP]

Thai troops have joined the search for victims of deadly mudslides that engulfed homes in the flood-hit south, officials said Thursday, as naval boats rescued hundreds of tourists stuck on holiday islands.

At least 21 people have died after unseasonably wet weather deluged the homes and businesses of around a million people in what should be one of the hottest months of the year.

Around 150 troops with sniffer dogs moved in to search for the dead and missing in the southern province of Krabi after a mudslide, army spokesman Colonel Sunsern Kaewkumnerd said.

Victims were either swept away by the rising waters, or buried in mudslides as the unseasonably wet weather deluged the homes and businesses of around a million people in what should be one of the hottest months of the year.

Residents were left without electricity in many areas as the waters rose, while road, rail and air links to the southern region remain closed.

Tourists stranded

Efforts to reach trapped tourists appeared to be making headway as flights to Koh Samui returned to normal and the country's only aircraft carrier, Chakri Naruebet, joined the rescue effort.

The ship docked in Sattahip naval base early Thursday morning carrying 734 holidaymakers, 532 of them foreigners, who had been picked up on the islands of Koh Tao and Koh Phangan.

The navy said more frigates are expected to collect tourists from isolated islands in the Gulf of Thailand during the day.

Bangkok Airways said flights to Koh Samui were operating normally and predicted it would clear the backlog of stranded passengers.

Deputy Prime Minister Suthep Thaugsuban said ferries to Koh Samui and Koh Phangan had also resumed.

"But for small islands like Koh Tao the naval response was critical. Thousands of tourists were stuck there and there was not enough food and water, so we had to help them first," he said.

In all, 80 districts of eight provinces have been declared disaster areas by Thailand's disaster prevention and mitigation department.

Devastating flooding across Thailand late last year left more than 220 people dead, damaging the homes or livelihoods of an estimated 8.6 million people in 51 of the kingdom's 76 provinces.