The World Health Organization (WHO) has raised its alert level over swine flu from three to four - two steps short of declaring a full pandemic.

世界卫生组织已经提高了对于猪流感的预警程度，将其从三级警戒提高到四级警戒，只差两级就可以达到最高预警。

WHO flu expert Dr Keiji Fukuda: "Containment¹ is not a feasible operation"

WHO Assistant Director General Dr Keiji Fukuda said it signalled a "significant step towards pandemic influenza²", but added "we are not there yet".

Mexico earlier said it believed 149 people had now died from the swine flu outbreak - only 20 cases are confirmed.

Other, milder, cases are confirmed in the US, Canada, Spain and Britain.

The WHO's decision to raise the alert level to four came after an emergency meeting of experts, brought forward by a day because of concerns over the outbreak.

Level four means the virus is showing a sustained（持久的） ability to pass from human to human, and is able to cause community-level outbreaks.

"What this can really be interpreted as is a significant step towards pandemic influenza. But also, it is a phase that says we are not there yet," Mr Fukuda said.

"In other words, at this time we think we have taken a step in that direction, but a pandemic is not considered inevitable³."

He said the virus had become too widespread to make containment（可容度，控制） a feasible（可行的，可能的） option, and said countries must focus on trying to put measures in place to protect the population.

He also stressed that the experts did not recommend closing borders or restricting travel. "With the virus being widespread... closing borders or restricting travel really has very little effects in stopping the movement of this virus," he said.

The first batches⁴（批，成批） of a swine flu vaccine⁵（疫苗） could be ready between four to six months, but it will take several more months to produce large quantities of it, Mr Fukuda said.

Health experts say the virus comes from the same strain that causes seasonal⁶ outbreaks in humans. But they say this newly-detected version contains genetic⁷ material from versions of flu which usually affect pigs and birds.

The BBC talks to people in Mexico City about the flu outbreak.

Mexico deaths

Earlier, Mexico's Health Minister Jose Angel Cordova said the suspected death toll⁸（死亡人数） from swine flu had now risen from just over 100 to 149. Of that number, 20 have been confirmed as swine flu.

All of those who had died were aged⁹ between 20 and 50, he said. Infections among young healthy adults was a characteristic of past pandemics.

"We're in the decisive moment of the crisis, the number [of deaths] will continue rising," Mr Codova told a news conference.

He said nearly 2,000 people had been hospitalised since the first case of swine flu was reported on 13 April, but half had now been allowed home.

Schools nationwide are to remain closed until 6 May as the country attempts to grapple with（格斗） the outbreak.

As Mr Cordova spoke¹⁰, Mexico City - where the outbreak is centred - was rocked by a 5.6-magnitude（震级） earthquake. It shook tall buildings and led to evacuations（撤离，疏散）, but there have been no reports of damage or injuries.

In almost all swine flu cases outside Mexico, people have been only mildly ill and have made a full recovery.

In the US, a further 20 cases of swine flu were confirmed in New York. Cases have also been reported in Ohio, Kansas, Texas and California, bringing the total across the country to more than 40.

It is thought that only person in the US had been hospitalised as a result of contracting（收缩的） the virus, and all had recovered.

Dr Richard Besser, acting¹¹ director of the US Centers of Disease Control and Prevention (CDC), has warned that a new US travel advisory¹² is being prepared suggesting "non-essential travel to Mexico be avoided".

Earlier, US Secretary of State Hillary Clinton and EU Health Commissioner¹³ Androulla Vassiliou separately urged caution for those considering travelling to Mexico.

In Canada, six cases have been recorded at opposite ends of the country, in British Columbia and in Nova Scotia.

Swine flu officially arrived in Europe on Monday, when tests confirmed that a young man in Spain and two people in Scotland - all of whom had recently returned from Mexico - had the virus. They were said to be recovering well.

Tests are also being carried out on individuals or groups in New Zealand, Australia, Brazil, Israel and South Korea who fell ill following travel to Mexico.

A number of countries in Asia, Latin America and Europe have begun screening airport passengers for symptoms, while Germany's biggest tour operator has suspended trips to Mexico.

Several countries have banned imports of raw pork and pork products from Mexico and parts of the US, although experts say there is no evidence to link exposure to pork with infection.

Shares in airlines have fallen sharply on fears about the economic impact of the outbreak.