Sing Buri Governor Surapol Sawangsak said three districts were flooded - Muang, In Buri and Phrom Buri. Water was up to a metre deep in some areas.
Mr Surapol said he had instructed local officials to make sure flood relief supplies get through to the people who need it. People were advised to closely watch for a rise in the floodwater.
Narongsak Wingwon, mayor of Muang municipality in Sing Buri, said the Chao Phraya dam was now releasing water at a quicker rate of 2,212 cubic metres per second, causing water levels to rapidly rise and flood villages in the municipality.
Officials had closed local waterways to prevent floodwater from entering the commercial areas of Muang district, he said.
In Ang Thong's Muang district, the flood situation remained critical with flood levels above 30 centimetres in places.
More than 100 households were flooded. Residents had to move their belongings to higher ground and officials were helping the elderly.
National Disaster Warning Centre (NDWC) chief Somsak Khaosuwan insisted most of Bangkok is safe from flooding.
Gp Capt Somsak said the level of the Chao Phraya river was not as high as expected, thanks to faster drainage, up from the usual 1,800 cubic metres per second to 2,001 cubic metres/second.
However, floodwater might flow into some areas outside the flood barriers, such as Pak Kret and Rangsit, he said.
"The overall situation is very different from the floods in 2011 as the volume of water is smaller.
"You can be at ease because there'll be no flooding in Bangkok this year. Even if two storms are approaching, we'll be able to handle them," Gp Capt Somsak said.
As for the severe flooding in Prachin Buri's Kabin Buri district, the situation should return to normal within a week, he said. Water levels in the area had already decreased.
There could also be storms in southern Thailand by the end of October and there might be flooding in the region, he added.
Chatchai Promlert, director-general of the Disaster Prevention and Mitigation Department, said the death toll from the floods remained at nine since Tuesday.
The number of flood-stricken provinces had risen from 23 to 27, he said.
In Kanchanaburi and Chumphon, the flood water had drained and the situation had returned to normal, he said.