Massive floods have left 500 people dead across Thailand, Cambodia and Vietnam, officials said Monday, as authorities stepped up efforts to reach victims of the unusually heavy monsoon rains.

In Thailand, where the death toll from the country's worst floods in decades rose to 269, thousands of soldiers fanned out across affected areas as part of a huge aid operation.

Prime Minister Yingluck Shinawatra, who has described the situation as a "serious crisis," said the kingdom had two days before the arrival of the next tropical depression, but insisted the situation was under control.

"It is not necessary to announce disaster zones because we still can handle it," she told reporters, a day after postponing official visits to Singapore and Malaysia to stay and monitor the authorities' response.

She said new flood defences would be built in several locations in the north and east of the capital.

In neighbouring Cambodia, the toll from the country's worst floods in over a decade reached 207, including 83 children, a disaster official there said. Vietnam has reported 24 deaths from flooding in the Mekong Delta.

Vast swathes of rice paddy have been damaged or destroyed in Southeast Asia as a result of the floods.

In Thailand the floods have damaged the homes or livelihoods of millions of people, particularly farmers, across about three quarters of the country's provinces.

Huge efforts are now under way to stop the waters from reaching low-lying Bangkok, home to 12 million people, with prevention measures including sandbags along the Chao Phraya river.

"We're confident that Bangkok is still in control. The situation is normal," said Narong Jirasubkunakorn, a senior official at the Bangkok Metropolitan Administration.

"We have 113 monitoring stations watching the overflow situation. As of now, every station reported a normal level."

He said water was being allowed through the city's canals and pumped out to sea to try to ease the situation in provinces north of the capital that have been badly affected, with water several metres deep in places.

In Thailand's ancient capital Ayutthaya, about 80 kilometres (50 miles) upriver of the capital, historic temples have been swamped and a large industrial estate, home to a slew of Japanese electronics and auto parts makers including car giant Honda, has been flooded.

A large amount of run-off water is expected to reach Bangkok in mid-October, while high tides will make it harder for the floods to flow out to sea, but the authorities said they were confident they could cope.

"The time we will have to watch carefully is the middle of the month and around the end of the month when the sea level will be high, but I think Bangkok will be just fine," said Narong