Floodwaters that have inundated the outskirts of Bangkok for several weeks are slowly spilling into the heart of the Thai capital, as a mass exodus of frightened residents continues.

Ankle-high water has been seeping into the city's iconic Grand Palace from surrounding streets that have been flooded due to the rising levels of the Chao Phraya River. The river is expected to burst its banks later this week because of unusually high tides in the Gulf of Thailand.

People have been piling sandbags outside their homes and businesses, while supermarkets are either empty or rationing bottled water and other essentials.

Roads leading out of Bangkok are jammed with cars filled with residents taking advantage of a government-issued five-day holiday, while thousands more are packed into bus terminals and the main airport's departure lounge, waiting for a chance to escape the floods.

Meanwhile, residents in many hard-hit provinces north of Bangkok have been wading through waist-deep flooded streets in makeshift boats.

The disaster has forced the closure of Bangkok's second-largest airport, the Don Muang airport, which mainly handles domestic flights.

The floods have been moving south towards central Bangkok along the Chao Phraya River and through a series of canals built to channel water out to sea.

An emotional Prime Minister Yingluck Shinawatra told reporters Thursday that all her government can do is try to manage to situation and let nature take its course.

Unusually heavy monsoon rain started flooding Thailand in July and has killed at least 373 people so far.