By Clare Arthurs BBC correspondent in Hanoi

Officials in the Vietnamese capital, Hanoi, say the 500-year-old dyke system which protects the city from floods is in danger of being breached.

Vietnam's annual flood season has started and the authorities are predicting it could be as bad as last year, when about 400 people died.

Throughout most of the year, the dykes around Hanoi provide a place to graze livestock or erect a thatched hut, ride a bike or grow a small crop.

But now the red-brown waters of the Red River are lapping the trees and Vietnamese families living within the dykes have gone to stay with relatives on dry land.

There are fears for the country's rice crop

And things are about to get a lot worse.

The government's disaster management unit says Hanoi has not had a major flood for decades but early in this year's flood season it has announced that water levels are reaching Alarm Level Three.

That means flood conditions are dangerous, dykes are threatened and the infrastructure is being damaged.

Prime Minister Phan Van Khai has been meeting emergency officials and highlighted the importance of the Hoa Binh reservoir, the main dam which protects Hanoi from flooding.

The gates have been raised to ease the pressure on the dykes but that reduces the ability of the dam to hold more flood waters.

There is also the risk of flooding from the Red River's main tributaries.

In theory, if the dykes break, the whole of Hanoi will be flooded.

But the authorities have other stand-by plans.

The ancient bed of the Day River could be used to divert flood waters, although 500,000 people would first have to be moved to safety.

There are also the traditional flooding areas - a network of natural depressions beyond the city which could be used as retarding basins.

The state media says almost 30 people have died in recent days from flooding in the northern, central and central highland regions of Vietnam.

In the south, the Mekong river has already reached last year's August heights, reinforcing predictions that damage in the major rice growing areas could again be serious.