New cases of the deadly swine flu virus have been confirmed as far afield as New Zealand and Israel, as the UN warns it cannot be contained.

The US, Canada, Spain and Britain confirmed cases earlier but no deaths have been reported outside Mexico, where the virus was first reported.

Mexico has raised the number of probable deaths to 152, with 1,614 suspected sufferers under observation.

UN inspectors are to examine reports that pig farms spread the virus.

The UN's health agency, the World Health Organization, said the flu was being spread by human-to-human transmission but did not recommend travel restrictions or border closures.

EU Health Commissioner Androulla Vassiliou has said she does not see "any point of restricting travelling".

WHO spokesman Gregory Hartl confirmed that work was under way to produce a vaccine, adding that it would typically be five to six months before it became commercially available.

Health experts say the virus comes from the same strain that causes seasonal outbreaks in humans but also contains genetic material from versions of flu which usually affect pigs and birds.

Students infected

In New Zealand, Health Minister Tony Ryall confirmed that at least three students who had travelled to Mexico were suffering from swine fever.

CONFIRMED OR SUSPECTED CASES Mexico: 152 suspected deaths from swine flu - 20 confirmed cases US: 50 confirmed cases Canada: 6 confirmed cases New Zealand - 3 confirmed cases UK - 2 confirmed cases Spain: 2 confirmed caseS Israel - 1 confirmed case Countries with suspected cases: Brazil, Guatemala, Peru, Australia, and South Korea, and seven EU states

Israel's first patient with confirmed swine flu, a 26-year-old man who recently returned from Mexico, is said to be making a good recovery.

Fifty cases have been confirmed in the US and six in Canada.

Two swine flu cases have been confirmed in the UK and two in Spain.

The EU health commissioner said on Tuesday that patients were also under observation in Denmark, Sweden, Greece, the Czech Republic, Germany, Italy and the Irish Republic.

Tests are being carried out on individuals or groups in Brazil, Guatemala, Peru, Australia and South Korea.

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.

Pig farm theory

WHO PANDEMIC ALERT PHASES Phase 1: No viruses circulating among animals causing infections in humans Phase 2: Animal influenza virus causes infection in humans, and is considered potential pandemic threat Phase 3: Influenza causes sporadic cases in people, but no significant human-to-human transmission Phase 4: Verified human-to-human transmission able to cause community-level outbreaks. Significant increase in risk of a pandemic Phase 5: Human-to-human transmission in at least two countries. Strong signal pandemic imminent Phase 6: Virus spreads to another country in a different region. Global pandemic under way

A team from the UN Food and Agriculture Organisation is due to leave Rome for Mexico on Tuesday.

Its chief veterinary officer, Joseph Domenech, told the BBC that rumours that people had been falling ill last month near some intensive pig farms meant the FAO had to act.

"[We had] no indication of human cases of direct contact with pigs but this can never be totally sure and the probability that this virus could come from pigs directly could not be anyway ruled out totally," he said.

The WHO has raised its alert level to four, or two steps short of a full pandemic.

Alert level four means the virus is showing a sustained ability to pass from human to human and is able to cause community-level outbreaks.

WHO deputy chief Keiji Fukuda said this was a "significant step towards pandemic influenza" but a pandemic should not be considered inevitable.

"With the virus being widespread... closing borders or restricting travel really has very little effects in stopping the movement of this virus," he added.

Hugh Pennington, a leading bacteriologist in the UK, said on Tuesday that it was difficult to make any predictions about the outbreak because of its nature.

"It's a new virus - we've never before seen this combination of swine virus and human virus genes," he said.

'Decline in cases'

In Mexico, swine flu has been confirmed in 20 of the 152 known deaths.

SWINE FLU Swine flu is a respiratory disease thought to spread through coughing and sneezing Symptoms mimic those of normal flu Good hygiene like using a tissue and washing hands thoroughly can help reduce transmission

Health Minister Jose Angel Cordova said all of those who had died were aged between 20 and 50. Infections among young healthy adults are a characteristic of past pandemics.

He noted that the number of new cases reported by Mexico's largest government hospitals had declined during the past three days: from 141 on Saturday to 119 on Sunday and 110 on Monday.

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

Source: BBC News