More rain lashed Vietnam's flood-hit capital Hanoi and the north-central countryside on Tuesday as the official death toll climbed to 74 after more than a week of heavy downpours.

Thousands of people, including children and the elderly, remained trapped in their water-logged homes in Hanoi, where 20 people have died since last Friday in what officials now call the capital's worst floods in 35 years.

Authorities worried whether the rain-soaked dyke system around Hanoi and across the northern Red River delta would hold back swollen waterways and have deployed thousands of troops to stand by for emergency repairs.

Across Hanoi, 44 neighbourhoods remained under dirty brown floodwaters up to 2.5 metres (8.3 feet) high, raising fears about outbreaks of diseases such as cholera and dengue fever, authorities said.

Hanoi officials said more than 9,000 troops had joined rescue efforts and over 5,000 households had received help -- but in many flood areas residents said there was hardly a police officer, soldier or rescue volunteer in sight.

Schools across the capital remained closed Tuesday, and hospitals were crowded with cases of respiratory and gastrointestinal disease.

Many districts still had no electricity and suffered shortages of drinking water, while food and petrol prices have multiplied in local markets.

Among those killed in Hanoi were 12 people who were swept away in floods or fell into open drains hidden under flooded roads, four victims of electrocution and two people killed by lightning, said authorities.

Across the disaster region, more than 120,000 buildings have been flooded, 250,000 hectares (over 600,000 acres) of rice and other crops have been lost, and 170 kilometres (105 miles) of rural roads damaged, officials said.

Television reports showed rescue workers using trucks to clear boulders and rubble from roads in remote northern mountain areas, where heavy rains in deforested areas routinely cause flash floods and landslides.

The worst-hit rural province was north-central Nghe An, where authorities have reported 22 deaths, including a 49-year-old man who was killed after saving three other people from drowning.

In Ninh Binh province, south of Hanoi, workers used hundreds of sand bags to reinforce a river dyke but thousands of houses remained under water, said an official with the flood and storm prevention committee.

Fifteen more people died in Ha Tinh province, and casualties were reported in provinces as far south as Quang Binh, a central region which has suffered almost two weeks of rains and floods.

Weather forecasters predicted more downpours in the north, while Hanoi authorities said it would take at least until the end of the week before flood waters could be pumped out of the worst-hit areas.

In neighbouring China, massive downpours in the southwestern Yunnan province and Guangxi region had also killed 43 people, mostly in landslides, and left 47 more missing by early Tuesday, state media reported.

Vietnam, a country of 86 million, gets lashed by typhoons and tropical storms every year, mostly along the central coast.

Last year, seven major storms from the South China Sea battered Vietnam, killing more than 435 people in floods and landslides, displacing thousands and leaving vast central areas inundated for months.