As of 7am, Typhoon Wutip, centred 480 kilometres east of the Vietnamese coast, was moving in a northwesterly direction at a speed of 10 kilometres per hour, with a speed of about 130kmh at the centre, the department said.
It is expected to make landfall over Hue, Vietnam, on Monday or Tuesday. Thereafter, it's strength will decline and move westward to Nakhon Phanom province.
The storm is expected to bring more rain, isolated heavy rain and gusts over Nakhon Phanom, Mukdahan, Kalasin, Nong Khai, Sakon Nakhon, Udon Thani, Nong Bua Lamphu, Loei, Khon Kaen, Chaiyaphum, Phitsanulok and Phetchabun, beginning with the eastern part of the Northeast on Monday afternoon.
Chatchai Promlert, chief of the Disaster Prevention and Mitigation Department, said more than two million people in 210 districts of 30 provinces have been affected by floods since Sept 17.
Mr Chatchai said the flood situation in five provinces had eased.
According to a report by the water situation analysis centre of the Royal Irrigation Department, as of Sept 27 the combined volume of water in all major dams around the country was 47.26 billion cubic metres, or 67% of capacity.
The dams can still take an additional 22.8 billion cubic metres of water during the rainy season.
The Royal Irrigation Department's emphasis is on the management of water at various reservoirs in line with the volumes of rainwater in each locality to keep water for use in the dry season.
In Chaiyaphum's Muang district, heavy rain continued to fall and flood levels had increased to about 50 to 70 centimetres since Saturday night. Main roads in the district were inundated and impassable to small vehicles.
Prime Minister Yingluck Shinawatra on Sunday visited Prachin Buri's Kabin Buri district where flood levels reached two metres.
Ms Yingluck was travelling with Chartthaipattana Party chief advisor Banharn Silpa-archa, Transport Minister Chatchart Sithipan, Interior Minister Jarupong Ruangsuwan and government water management committee member Royol Jitdon.
The premier and her entourage distributed relief supplies to flood victims and discussed ways to deal with the flood situation.
In Prachin Buri's Si Maha Phot district, supporters of the red-shirt United Front for Democracy against Dictatorship (UDD) gave out small boats, packed rice and dried food to people affected by flooding.
Sa-nga Khunyosying, a red-shirt leader in Prachin Buri, said 100 small boats and 2,000 bags of packed rice and dried goods were given to locals in the area.
Ms Sa-nga said the relief supplies were from Thaksin Shinawatra. The ousted premier wanted to help people who have been hit by flooding, she said.