Typhoon Haiyan made landfall in Vietnam early Monday, meteorologists said, days after it left thousands feared dead and widespread devastation in the Philippines.

The US Joint Typhoon Warning Center (JTWC) said in an update at 2100 GMT the storm "is currently making landfall" approximately 100 miles (160 kilometres) east south-east of the capital Hanoi.

The storm, which had weakened significantly since scything through the Philippines over the weekend, made landfall with sustained winds of 75 miles (120 kilometres) per hour, said the JTWC, a joint US Navy and Air Force task force located in Pearl Harbor, Hawaii.

More than 600,000 people were evacuated on the weekend as Haiyan bore down on Vietnam.

Residents of Hanoi were braced for heavy rains and flooding, while tens of thousands of people in coastal areas were ordered to take shelter.

"We have evacuated more than 174,000 households, which is equivalent to more than 600,000 people," said an official report by Vietnam's flood and storm control department.

The storm changed course on Sunday, prompting further mass evacuations of about 52,000 people in northern provinces by the coast.

"People must bring enough food and necessities for three days.... Those who do not move voluntarily will be forced," online newspaper VNExpress said, adding all boats have been ordered back to shore.

The Red Cross said Haiyan's changed path meant that "the disaster area could be enlarged from nine provinces to as many as 15", stretching the country's resources.

Many of the capital's residents were rushing to stock up on food and water before the storm hit.

"I ran to the supermarket to buy instant noodles, vegetables and meat for the family," said office worker Nguyen Thi Uyen, 33.

"There was not much left on the shelves.... People are worried, buying food to last them for a few days."

All schools were ordered shut in the capital Monday and extra police were dispatched to redirect traffic in flood-prone areas.

In the northern port city of Hai Phong, also facing heavy rain and flooding, residents voiced frustration with official preparations.

"The city only warned us about the typhoon very late.... They were too slow in advising people to prepare," Nguyen Hung Nam, 70, told AFP.

Many of the estimated 200,000 people evacuated in four south-central provinces initially thought to be in the storm's path have been allowed to go back to their homes, according to the government's website.

Haiyan "has tracked north-northwestward at 15 knots (17 mph, 28 kph) over the past six hours," the JTWC said on its website.

The storm was forecast to continue moving north before turning northeast and dissipating rapidly.

The weather system -- one of the most intense typhoons on record when it tore into the Philippines -- weakened over the South China Sea.

In Vietnam, at least five people reportedly died while preparing to escape the typhoon, the Vietnamese government website said.

By lunchtime on Sunday the typhoon had swept across Vietnam's Con Co island, 30 kilometres off the coast of central Quang Tri province, the Tuoi Tre newspaper reported.

"All 250 people on the island including residents and soldiers were evacuated to underground shelters where there is enough food for several days," it said, adding the storm brought three-metre (10-foot) waves.

Central Vietnam has recently been hit by two other typhoons -- Wutip and Nari, both category-one storms -- which flooded roads, damaged sea dykes and tore the roofs off hundreds of thousands of houses.