The rains have raised the water level at the Bui River, 20 km from downtown Hanoi, to an "alarming level", and threaten nearby villages, the Hanoi People's Committee said in a statement published on its website on Tuesday.

"The authorities must standby around the clock to ensure the safety of local people and their properties," committee chairman Nguyen Duc Chung said in the statement.

"We must be prepared for the worst-case scenario," said the statement from the committee, which functions as the city's executive branch, or mayoral office.

Vietnam is prone to destructive storms and flooding, with 389 people killed last year in natural disasters such as floods and landslides, according to government statistics.

Two children have drowned and more than 1,000 houses in the Hanoi district have already been badly flooded, the statement said. Some 4,000 people have already been evacuated.

The rain is expected to ease on Wednesday, according to Vietnam's National Centre for Hydro-Meteorological Forecasting.

Last week, floods and landslides triggered by tropical storm Son Tinh killed at least 27 people and left seven people missing in other northern Vietnamese provinces.

More flooding is expected in the coming months until the rainy season comes to an end around early October.

REUTERS