The water level continued to rise on Monday and spread into the inner areas of the district.
Several schools were closed temporarily while vendors in several flooded markets moved their stalls out onto the main street, causing heavy traffic congestion.
Authorities said the water level in the Yom River had fallen by 10cm, but more than 1,400 residents in Sri Samrong district continued to struggle against the flood tide, which had reached over two metres deep in some areas.
Locals had begun building sandbag walls in their villages to protect their houses from further inundation.
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Provincial authorities in Phitsanulok expect the flood surge moving downstream from Sukothai province will reach three northern districts by Tuesday and the flood levels there could rise by 50cm to one metre. The three districts are Bang Rakam, Porn Prompiman and Muang districts.
According to several reports, the flooding had expanded in many areas. Several main roads leading to villages in Porn Prompiman district had been cut and local people were using boats to move around the area. At least 100 houses had been flooded while about 5,000 rai of farmland was already under a metre of flood water.
In Bangkok's neighbouring province of Pathum Thani, the Chao Phraya River overflowed its banks and flooded many houses and schools in Muang district.
Reports said the water in one school, at Wat Pho Luen, was at least 40cm deep, forcing teachers and students to move to higher ground.
In Phichit province, Royal Irrigation official Prawes Sirisil said a large mass of water from Sukhothai and Phitsanulok had reached Phichit.
Mr Prawes said the Yom River in Phichit's Sam Ngam district was about four centimetres higher than usual.
He said sandbags had been brought in to reinforce the flood barriers.
Science and Technology Minister Plodprasop Suraswadi said the volume of water moving down from the North was much less than at this time last year, and the government's improved potential for flood management should ensure the situation remains manageable this year.
Mr Plodprasop, who is chairman of the government's Water and Flood Management Committee (WFMC), was confident the situation would remain manageable - unless the country was hit by as many as ten typhoons in just one month, which would be a very rare event.
He said at present a total area of about 850,000 rai in the lower North and upper Central regions had been flooded, only about one-third the area flooded last year.
He said the water in Sukhothai had begun to recede. Some areas along the river banks might still be flooded, but this was not worrying.
There would not be a prolonged flood like last year, Mr Plodprasop said.
He said the water was now flowing south through the lower North at 1,400 million cubic metres per second and was unlikely to exceed 5,000 million cubic metres per second, unlike last year when the average water flow was recorded as 24 billion cubic metres per second.
Moreover, this year's flooding was caused by the annual monsoon, not by storms.
He said the government would try to keep the public informed of the overall situation at least once every day.
The WFMC would from now on meet every day, issue a daily weather forecast at 10am and a daily water situation report at 3pm.
National army chief Prayuth Chan-ocha said people should not be alarmed about the flood situation because it would only lead to confusion and make the situation worse than it is.
"I ask the people not to be alarmed because all government agencies and the army are ready to provide immediate assistance," Gen Prayuth said.
"The people must also cooperate and try to understand, because if they're against everything how can we help?"
When reporters asked him about the necessity to construct Kaeng Suar Ten dam on the Yom River, he declined specific comment, but did mention that the public should consider ways to store water from the Yom River, particularly for the dry season.
"Flooding happens every rainy season and we should find ways to prevent flooding and to store water for the dry season. If you don't want a dam or water retention areas to be built, and if you're against everything, how else can we solve the problem?" the army chief said.
He said 100 companies of soldiers were ready to provide help in flood-affected areas.
Royal Irrigation Department director-general Lertwiroj Kowattana said the Yom River should return to normal levels in three days if there is no more rain in upstream areas.
"The overall water situation in the country is normal even though the water level is high in some areas because of a low pressure front," Mr Lertwiroj said.
He said the low pressure front and heavy rain in Phrae's Wang Chin district and Sukhothai's Si Satchanalai district had caused a rise in the water level of the Yom River. The flow rate of the Yom River at Sukhothai was more than 1,500 cubic metres per second, causing overflows in areas with low dykes.
"Authorities are now using pumps to drain the flood water and the situation should return to normal in the next few days if there's no more heavy rain," he said.