The Interior Ministry reported on Thursday that flood levels in seven provinces in upper Thailand were either stable or decreasing, except in Nan, where floodwaters were rising.
Traisuree Taisaranakul, secretary to the interior minister, said that the affected provinces included Chiang Rai, Lampang, Nan, Phayao, Phetchabun and Phrae in the North, as well as Udon Thani in the Northeast.
Officials were evacuating flood victims, distributing essential supplies and assessing damage.
According to Ms Traisuree, 12,777 families in 541 villages were affected by flooding.
Nan governor Chainarong Wongyai reported that floodwaters in central districts were rising at a slower pace as rain ceased, and water levels were declining in upper districts. Ten districts in Nan were still heavily inundated.
In Muang district, the Nan River reached a depth of 8.25 metres, just 25 centimetres below the district's embankment.
The governor urged residents living near waterways to move valuables to higher ground and prepare for evacuation, as deluges from northern areas were approaching.
The Office of National Water Resources (ONWR) warned of possible flooding and landslides in 35 provinces across all regions until Aug 30 due to heavy rains.
The Meteorological Department warned people in northern and northeastern provinces to be cautious of heavy to very heavy rains from Aug 21 to 27, which may lead to flash floods and overflow, particularly along foothills, near waterways and in low-lying areas. All ships in the Andaman Sea and the Gulf should proceed with caution in thundershower areas during this period.