The World Health Organisation warned today that swine flu can no longer be contained as the virus spread to Asia and the Middle East, with the first cases confirmed in New Zealand and Israel.

As Britain advised holidaymakers not to travel to Mexico, WHO spokesman Gregory Hartl said the world was now at a "turning period" where it would become clear whether the virus was capable of sustained movement between humans on a large scale, bringing the likelihood of a pandemic a step closer. So far community transmission had been seen in Mexico and the US, he said.

Hartl said it was not clear whether the British couple confirmed to have the virus, Iain and Dawn Askham, had both been infected in Mexico or whether one had infected the other.

Some flights to Mexico from the UK were cancelled this morning and arrangements were being put in place to bring British holidaymakers home after the Foreign Office advised against all but essential travel to the country. All Thomson and First Choice flights to Mexico from Manchester this week have been grounded until restrictions are lifted.

The 11 cases in New Zealand, among students and teachers from a single school who reported fevers and other flu symptoms upon return from a visit to Mexico, are the first in the Asia-Pacific region.

A 26-year-old patient in Netanya, a coastal city north of Tel Aviv in Israel, was infected but was said to have recovered fully.

Passengers due to fly to Mexico with Thomson this morning said they had been offered their money back or an alternative holiday. The company said it was making arrangements to get customers home from Mexico, with repatriation flights starting today.

The Foreign Office advised British citizens living in Mexico to "consider whether they should remain". The Mexican government has closed schools across the country.

Spain confirmed a second case of a patient with swine flu today and South Korea said a woman there had tested positive for swine flu, making her a "probable" case.

The prime minister, Gordon Brown, who will join a meeting of the government's Cobra emergency committee chaired by health secretary Alan Johnson today, said he would take "all urgent action necessary" to try to halt the virus.

Brown said he understood the "very real concern" of Britons but added that the UK was among the best prepared countries in the world.

Last night, the WHO's emergency committee raised the pandemic threat level to phase four out of a possible six, after the death toll in Mexico rose to 152, with nearly 2,000 people believed to be infected, the number of cases in the US doubled and the first infections were confirmed in Britain.

Phase four means that the crisis had taken a significant step towards pandemic, but does not mean one was inevitable. Nonetheless, the WHO said, the virus had spread so far that containment was "not a feasible operation" and the international response should be to try to limit its transmission and treat those affected.

The Scottish couple suffering from swine flu, from Polmont, near Falkirk had been in Mexico on honeymoon in Cancún.

About 22 friends, family and colleagues who had close contact with them were being observed and had been given antiviral drugs. Seven of these were showing "mild symptoms" of the illness. The group being monitored reportedly includes a five-year-old child.

Last night, the Scottish health secretary, Nicola Sturgeon, said the couple were recovering well at Monklands hospital in Airdrie, Lanarkshire.

Medics at Manchester airport treated a passenger arriving from Mexico this morning who had complained of feeling unwell on the flight, the Health Protection Agency said. She has been sent home and has been tested for swine flu.

The first confirmed case in Europe came yesterday in Spain, where 26 other cases are suspected. There are four suspected cases in the Irish Republic.

In an indication of the seriousness with which the threat is being taken in the UK, the Guardian has learned that if the situation deteriorates, Johnson is considering warning the entire population to set up a support network of friends and relatives, so they can be quickly quarantined at home if they are thought to have symptoms. The friends would then collect medicine on their behalf. He abandoned plans to give this advice as one of his four key messages yesterday in a Commons statement.

He told MPs it was too early to say if there was a pandemic, but the UK had been preparing for one for five years and had a stock of 33m anti-flu drugs. He said it was important to note that outside Mexico all those who had shown symptoms of swine flu had recovered.

Mexico's health secretary, José Ángel Córdova, said he expected more people to die. Nearly 2,000 people had been treated in hospital for suspected infection, he said. Half of them had been released.

Córdova admitted that the health authorities lacked the staff to check on all suspected cases. Some foreign health officials fear such difficulties may be contributing to the disease's spread.

In New York, the number of confirmed cases among students at a school rose to 28, with more than 100 suspected. That brought the number of confirmed cases in the US to 42 in five states, twice as many as reported at the weekend.

Peru and Guatemala reported the first suspected infections in other parts of Latin America, where health officials fear swine flu has already spread but so far gone undetected.



Gauging the spread: WHO viral infection phases

Phase one: No animal viruses circulating are causing infections in humans.

Phase two: An animal flu virus is known to have caused human infection and is considered a potential pandemic threat.

Phase three: Limited human-to-human transmission may occur. This does not indicate the virus has gained transmissibility that would cause a pandemic.

Phase four: Human-to-human transmission able to cause "community-level outbreaks". Significant increase in pandemic risk but it is not a foregone conclusion.

Phase five: Human-to-human spread of the virus into at least two countries in one WHO region. A strong signal that pandemic is imminent.

Phase six: Pandemic phase, characterised by community level outbreaks in at least one other country in a different WHO region along with phase five.