The Mekong River, which has been bloated with floodwaters from upstream provinces, burst its banks on Sunday morning, flooding the Muang district of Nong Khai province.
Officials and local residents said the river started overflowing at around 7.15am to flood vast areas of the province.
The overflowing caused the entire Prachak Sila Pakhom Road in the downtown area to be inundated. At some spots, the water level was more than a metre high.
The floodwaters on the bloated river continued to flow downstream to Bueng Kan and Mukdahan provinces. The waters mainly came from the Sai River that flowed into Mekong via the Ruak River. The Sai River, which originates in Myanmar, swelled following days of heavy downpour in Myanmar and Chiang Rai.
Local officials started dispatching flat-bottomed boats to inspect flooded communities and evacuate them as the water was still rising.
The Muang Nong Khai Municipality reported that at 7am the water level on the Mekong River measured 13.81 metres.
The Mekong River, which flows from China, enters Thailand in Chiang Saen district of Chiang Rai province and flows past Chiang Khong district before entering Laos.
From Laos, it flows into Chiang Khan district of Loei province and continues to Nong Khai, Bueng Kan, Nakhon Phanom Mukdahan, Amnart Charoen and Ubon Ratchathani provinces.
The massive floodwaters are expected to reach Cambodia and Vietnam by Tuesday and Thursday respectively.