* WHO awaits U.S. confirmation on sustained human spread
* If confirmed, phase 5 pandemic alert could be declared
* Questions remain on low mortality in Mexico
* Worldwide, cases confirmed to WHO rise to 79 with 7 deaths
(updates with Fukuda in paragraphs 7-9; new figures paragraph 16)
By Stephanie Nebehay
GENEVA, April 28 (Reuters) - The World Health Organisation said on Tuesday it awaited formal confirmation from U.S. authorities that the new swine flu virus has spread significantly between people, a sign that could indicate an "imminent" influenza pandemic.
Confirmation that infected people in two countries are spreading the new disease to their families or contacts in a sustained way would meet the WHO's criteria for declaring a phase 5 alert on its scale of 1 to 6.
The U.N. agency raised its pandemic alert level to phase 4 from phase 3 on Monday as the virus jumped to Europe.
The U.S. Centers for Disease Control and Prevention (CDC) said on Monday some people who have had contact with confirmed cases were also developing flu-like symptoms.
"We're still awaiting for a final confirmation from the U.S. authorities, but it appears that there's a number of cases in New York which appear to be human-to-human transmission," WHO spokesman Gregory Hartl told a news briefing.
Such secondary transmission of the virus was "probable", he later told reporters, adding: "If we have a confirmation from the United States or Canada, we could move to phase 5."
U.S. authorities are studying an outbreak in New York where many schoolchildren have caught the disease after returning from a trip to Mexico.
Keiji Fukuda, acting WHO assistant director for health security and environment, said it was possible that the virus could spark widespread infection in areas of multiple countries -- setting off what could prove to be a "very mild pandemic".
"In New York City, some of the studies show people who became ill did not travel, again suggesting the virus has the ability to have person-to-person transmission, to transmit from travellers to people who did not travel," he told reporters.
"So there is definitely the possibility that this virus can establish that kind of community-wide capacity in multiple countries. It is definitely a very serious possibility."
The emergency committee recommended moving to phase 4 on Monday, but WHO has not announced any future meetings. Its experts could convene at any time to make such decisions.
People have occasionally caught swine flu from a pig but it has stopped there. Avian influenza has occasionally spread from one person to just one other person and stopped there.
But the new H1N1 flu appears to be spreading beyond that limited chain, which is what worries the WHO.
More than 15 WHO epidemiological experts have been deployed in Mexico to help authorities combat the outbreaks in Mexico City and other areas.
They are helping to collect new data and ensure that virus specimens are tested to shed new light on the epidemic which Fukuda hoped would be forthcoming in the near future.
So far, 26 swine flu cases in Mexico have been confirmed by laboratory testing, including seven deaths, according to WHO.
In all, the WHO has official laboratory confirmation of a total of 79 cases of the new swine flu virus worldwide. They include 40 confirmed cases in the United States, 26 in Mexico, 6 in Canada, 2 in Spain, 2 in Britain and 3 in New Zealand.
The U.S. Centers for Disease Control said on Tuesday it had confirmed a total of 64 human cases of swine flu across the country.
The WHO's official total does not include 2 cases which have been announced by authorities in Israel. The Geneva-based agency requires formal legal notification of cases.
Mexican authorities say the new strain has killed up to 149 people in Mexico, but are still investigating.
A central question is why Mexico is the only country so far where deaths have been confirmed, although the mortality rate has been relatively low in relation to the number of cases, while elsewhere infections have been mild.
"We don't understand why the disease has been more severe in Mexico," Hartl said. (For more Reuters stories on swine flu, click on [nFLU]) (For more Reuters swine flu coverage, please click here:
) (For WHO information on swine flu, go to:
) (Editing by Jonathan Lynn and Sophie Hares)