Close to a million Filipinos have been, or may have to be evacuated from their homes as the country braces for the storm.

Mangkhut's damage is not expected to be limited to the Philippines, with preparations underway in other parts of south-east Asia ahead of the typhoon making landfall in mainland China, Vietnam and Hong Kong.

How the Philippines' central government handles the super typhoon - and the massive clean up effort that will likely be required afterwards - also looms as a major test for President Rodrigo Duterte, whose long political honeymoon has definitively ended in recent months amid political scandals and economic difficulties.

The typhoon has been tracked in the Pacific ocean with winds of up to 255 kilometres per hour, and sustained winds of about 205km/h, according to Philippine forecasters.

The US military's Joint Typhoon Warning Centre has forecast that wind gusts could reach 333 km/h.

Authorities fear the winds will cause a significant number of casualties, and anticipate the clean up could take months.

A huge rain cloud band 900- kilometres wide and heavy monsoon rains will accompany the huge winds, potentially setting off landslides and flash floods, and residents are already preparing their homes for when the typhoon makes landfall.

It is approaching at the start of the rice harvesting season and the provinces it is expected to hit, including Cagayan and Isabela, are major food producing regions in the north of the country. Farmers are already trying to save what they can of their crops before the typhoon hits.

After the Philippines, the typhoon is expected to slow down somewhat to winds of around 175km/h and to hit mainland China, while Hong Kong and the Vietnamese capital of Hanoi are also likely to feel some of the storm's effects.

Many flights have already been cancelled, Filipino government agencies have swung into action to prepare for the worst and international aid agencies are already flagging the need for donations to help with rebuilding after the storm hits on Saturday morning.

The typhoon is the 15th to hit the country this year and is expected to be the strongest. It is also expected to be the worst since typhoon Haiyan five years ago, which killed more than 6300 people.

Duterte has asked Cabinet officials from the north to help oversee disaster-response work if needed, and told reporters it was too early to consider seeking foreign aid.

"It would depend on the severity of the crisis," Duterte said. "If it flattens everything, maybe we need to have some help."

Philippines political analyst and author Richard Heydarian said that, in the wake of typhoon Haiyan, Duterte and his political allies had strongly criticised then president Benigno Aquino for his handling of the diaster.

Now, the tables have turned and Duterte has looked flat-footed in his handling of the looming disaster, as he had done in responding to a typhoon last month. He seems distracted by a faltering economy and a political fight with Senator Antonio Trillanes IV and other members of the opposition.

"He should be on the ground mobilising people, but the problem is the President himself is a bit complacent. It's likely many communities will be devastated by this typhoon. Now, the opposition can potentially return the favour from Haiyan."

UNICEF Australia Chief Executive Tony Stuart said the organisation was monitoring developments closely and was ready to act if the Philippine government asked for help.

Hundreds of thousands of children were in the path of the storm, he said, and were likely to be the most vulnerable to its impact.

- with AP