Flooding has killed at least 25 people in Thailand’s south since the start of the year, affecting more than a million people.
The Interior Ministry said on Tuesday that the floods have hit 12 provinces since January 1, cutting off the south from the rest of the country after the main highway connecting the region with the other parts was swamped.
A highways department spokesman said vehicles were stopped from passing the highway after two bridges collapsed. The rising floodwaters have also suspended train services south.
The death toll has risen over recent days as floods have reached roof-top level in some areas.
A five-year-old girl was among the victims. She lost her life in Prachuab province late on Monday after a flash flood hit a van she was travelling in.
"Her family climbed to the roof of the van to avoid the water but she fell in with her mother," relief worker Rawiroj Thammee said.
The girl was swept away and villagers found her body 200 meters from the van on Tuesday morning, he added.
Thailand's wet season usually ends in late November and heavy downpours are rare in January. Widespread floods killed over 900 people in the Southeast Asian state in 2011.
The region has been hit by what Thai Prime Minister Prayuth Chan-o-cha described as the heaviest January rainfall in 30 years.
The downpours have left schools closed. One of the major airports in the region has been sealed since Friday and will be closed until at least Wednesday.
The flooding has submerged vast tracts of the south, which is an agricultural hub for rubber, palm oil and fruit plantations.
Soldiers have been deployed to take part in the relief and rescue operations in the worst-affected areas.
The rain is predicted to slacken over the next 24 hours in the region, which is known as a major tourist destination.