Cambodia, with a quarter the population of its richer neighbour, is scrambling to provide aid for tens of thousands of families, with 1.2m people affected, AP reported. Around 2m have been displaced in Thailand.
With the inclusion of deaths in Vietnam's Mekong Delta, around 500 are thought to have been killed, CNN reported.
Over 80 children were among those killed in two months of flooding caused by heavy rainfall that has seen the Mekong River burst its banks, according to the National Committee for Disaster Management.
More than 270,000 families have seen their homes or livelihoods waterlogged by the floods, which have inundated 350,000 hectares (865,000 acres) of rice paddies across the country, said the committee spokesman Keo Vy.
The flooding, which has killed 207 people, including 83 children, is the deadliest since 2000 and represents a huge challenge to the impoverished nation.
Despite the scale of the disaster Keo Vy insisted the situation was under control.
"We are not yet appealing for international assistance," he said, adding that the waters have begun to subside.
But he said the floods were "very serious for us", AP reported.
The government, the Red Cross and several other relief organisations are working to provide aid, including urgent food deliveries, to the flood victims, he said, reaching some 67,000 families so far.
Cambodia has also received an emergency aid shipment from Japan, which included tents, blankets and water containers.
Officials estimate the damage to rice crops and infrastructure has reached at least $95 million, Keo Vy said.
BANGKOK BRACES
In neighbouring Thailand, the worst monsoon floods in decades are seeing authorities rushing to build sandbag barricades in the capital, Bangkok
Prime Minister Yingluck Shinawatra said on Monday that government workers have two days to build three major water barricades before runoff from the north reaches the city, AFP reports.
Unusually high ocean tides are expected to worsen the floods.
The Disaster Prevention and Mitigation Department said 269 people have died, mostly from drowning, since tropical storms began hitting Thailand at the end of July. It said 8.2 million people in 60 of the country's 77 provinces have been affected by floods and mudslides, and 30 provinces are now inundated.
Yingluck said she didn't know if Bangkok would be protected from the flooding.
"It is really hard to tell because it's difficult to predict the volume of water," she told reporters. "But I insist if we can complete the three main water barriers within the next one or two days, Bangkok will be safe."
The government planned to use 1.5 million sandbags to build the barriers but still lacked more than 100,000 as of Monday.
In Ayutthaya province to the north, flooding forced more than 200 factories in two industrial zones to shut, including Japanese automobile giant Honda, whose production plant suffered water damage.
