A deadly swine flu virus first detected in Mexico can no longer be contained, a World Health Organization (WHO) official has said. WHO Assistant Director General Keiji Fukuda said countries should now focus on mitigating the effects of the virus. The WHO has raised its alert level from three to four, two steps short of declaring a full pandemic. The number of probable deaths from the virus in Mexico has officially risen to 152 though only 20 cases are confirmed. The US, Canada, Spain and Britain have confirmed milder versions. 'Not inevitable' 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 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 inevitable." He said the virus had become too widespread to make containment a feasible 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 batches of a swine flu vaccine could be ready in four to six months' time, 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 seasonal outbreaks in humans. But they say this newly detected version contains genetic material from versions of flu which usually affect pigs and birds. Mexico deaths Earlier, Mexican Health Minister Jose Angel Cordova said the suspected death toll from swine flu had risen to 152. Of that number, 20 have been confirmed as swine flu. Reporting earlier, he said that all of those who had died were aged between 20 and 50. Infections among young healthy adults were a characteristic of past pandemics. "We're in the decisive moment of the crisis, the number [of deaths] will continue rising," Mr Cordova told a news conference. He said the first suspected case of swine flu had occurred in the southern state of Oaxaca. Nearly 2,000 people had been hospitalised since the first case of swine flu was reported on 13 April, he added, but half had now been allowed home. Schools nationwide are to remain closed until 6 May as the country grapples with the outbreak. In almost all swine flu cases outside Mexico, people have been only mildly ill and have made a full recovery. Travel advice 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. Dr Richard Besser, acting director of the US Centers of Disease Control and Prevention (CDC), warned that a new US travel advisory was being prepared suggesting non-essential travel to Mexico should be avoided. US Secretary of State Hillary Clinton and EU Health Commissioner Androulla Vassiliou have 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. Taiwan has announced it will quarantine any visitors showing symptoms of swine flu. 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. Source: BBC