Floods that have sparked an exodus from the Thai capital crept closer to the city centre on Friday as the government considered cutting through roads blocking the path of the water.

The city of 12 million people is on heightened alert because of threats on two fronts -- a seasonal high tide this weekend that is expected to coincide with the arrival of a mass of water from the flood-stricken central plains.

The three-month crisis -- triggered by unusually heavy monsoon rains -- has left at least 377 people dead and damaged millions of homes and livelihoods, mostly in northern and central Thailand.

Billions of cubic metres of water lie north of the capital, moving slowly southwards as the authorities attempt to channel the muddy brown liquid through the city's canals and rivers, but most of the city centre remained dry.

Some areas in northern Bangkok have seen waist-deep flooding, leading to the shutdown of the city's second airport, Don Mueang.

The government said it was considering a proposal by the private sector to dig temporary drainage channels through five roads blocking the flow of water in eastern Bangkok to speed up the flow into the Gulf of Thailand.

The authorities have opened sluice gates around the city to allow water through canals but experts and officials have given conflicting information about the risks of major flooding in downtown Bangkok.

While the government is largely focused on defending the capital, people in the worst-hit provinces north of the city have endured weeks of flooding.

Tens of thousands of residents have left Bangkok after the government declared a special five-day holiday, flocking to rail and bus stations in the city and jamming roads as they head to areas out of the path of the water.

So far, however, central Bangkok has only seen minor inundation in areas along the main Chao Phraya River, including near the Grand Palace, with the water receding after high tide passes.

Tourists walking through ankle-deep water near the Grand Palace appeared unfazed, despite a slew of travel warnings from foreign governments.

"It's adding to our experience," said 32-year-old British honeymooner Melanie Willoughby. "They all seem to be coping well. The only thing we found is that it's been hard to get (drinking) water."

Friday's high tide was lower than expected, raising hopes that the river's flood barriers would prevent a major overflow.

"The walls can still hold it back, despite flooding on the river banks which is usual during high tide," said an official at the city's Drainage and Sewerage Department who did not want to be named.

On Thursday an emotional Prime Minister Yingluck Shinawatra, in office for barely two months, warned that the country could not resist the "force of nature" by trying to hold back the water forever.

More than 100,000 people have sought refuge at emergency shelters and tens of thousands of troops have joined the relief efforts.

Almost 300 critically ill hospital patients have been evacuated from Bangkok to other provinces as a precaution.

The crisis is taking its toll on the lucrative Thai tourism industry, with the United States joining other countries including Britain, Singapore, Canada in advising against all but essential travel to Bangkok.

Most of the country's top tourist destinations have been unaffected by the disaster and Suvarnabhumi Airport, the main gateway to Thailand, is operating as normal, along with the city's subway and elevated train.

Thousands of inundated factories have been shut down, putting more than half a million people temporarily out of work.

Japan said Friday it would allow thousands of Thai workers employed by its companies affected by the floods to go and work there for six months.