More than 10,000 people are believed to be dead and millions left homeless after Typhoon Haiyan struck the Philippines with speeds of up to 170mph (274kph).
Major landslides have knocked out power and communications and causing widespread damage.
High school teacher Andrew Pomeda told AFP: "Tacloban is totally destroyed. Some people are losing their minds from hunger or from losing their families.
"People are becoming violent. They are looting business establishments, the malls, just to find food, rice and milk.
"I am afraid that in one week, people will be killing from hunger."
Tourists in northern areas of Vietnam are bracing themselves as the storm makes its way towards them after devastating the Philippines.
Britain has committed almost £6 million of emergency help to the devastated country.
A team of four humanitarian experts arrived in the disaster zone yesterday to join three already there in helping to co-ordinate the international response.
After leaving a trail of destruction the typhoon is now racing across the South China Sea towards the north eastern coast of Vietnam, and is expected to make landfall shortly after midnight.
More than 100,000 Britons visit Vietnam every year. Many travel to the northern city of Hanoi, the capital, and the popular tourist destination of Halong Bay in the north east of Vietnam, both of which are expected to be in the path of the typhoon.
The Foreign Office is advising travellers to follow advice from local authorities, and has teams on standby in Hue and Da Nang to assist any British nationals.
A second team of experts is on standby to travel there.
Alexi Boothman, a forecaster with MeteoGroup, the weather division of the Press Association, said the typhoon would decrease in power when it reached land, curving north eastwards through Vietnam.
She said: "Typhoons are powered largely by the warm ocean, so when they move over land they start to lose that energy and dissipate."
The typhoon is likely to hit Vietnam at speeds of about 75mph (120kph).
In the Philippines 6,000 foreigners and locals are stranded on the popular resort island of Boracay, one of the tourist spots in the typhoon's path, tourism officer Tim Ticar said.
It is not known if any Britons are trapped.
As many as 10,000 people are believed to have died in the city of Tacloban alone.
Ferocious winds have caused widespread flooding, washing away homes, schools and airport buildings, burying people under rubble and leaving millions homeless.
Aid agencies in the country say they are struggling to reach the worst-affected areas.
Yesterday International Development Secretary Justine Greening activated the UK's Rapid Response Facility (RRF) which will provide £5 million to organisations to help up to half a million victims.
Plastic sheeting to build shelters is among a £600,000 British shipment of emergency equipment.
Ms Greening said: "My thoughts are with the people of the Philippines, in particular those who have lost loved ones. UK support is now under way.
"Many thousands of people in remote, hard-to-reach communities have lost their homes and everything they own. They are living in the open and completely exposed to the elements.
"The absolute priority must be to reach them with shelter and protection as soon as possible.
"UK support will provide urgently needed access to clean water, shelter, household items and blankets.
"We are also sending additional humanitarian experts from the UK to work with the DfID team and international agencies, including ensuring partners are prioritising the protection of vulnerable girls and women."
The UK already funds emergency preparations in the Philippines, with almost a third of a recent £4.1 million tranche of aid being devoted to the issue, the Department for International Development said.