* WHO awaits U.S. confirmation on human-to-human spread

* If confirmed, phase 5 pandemic alert could be declared

* Questions remain on low mortality in Mexico

GENEVA, April 28 (Reuters) - The WHO said on Tuesday it awaited formal confirmation from U.S. authorities the new swine flu virus has spread significantly between people, a sign that could indicate an "imminent" influenza pandemic.

Confirmation infected people in two countries are spreading the new disease to their families or contacts in a sustained way would meet the World Health Organisation's criteria for declaring a phase 5 alert on its scale of 1 to 6.

The United Nations agency raised its pandemic alert level to phase 4 from phase 3 on Monday as the virus spread 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.

"It appears, and I think 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."

The emergency committee which recommended moving to phase 4 late on Monday -- a "turning point", according to Hartl -- was not scheduled to meet on Tuesday. But its experts could be convened 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.

CENTRAL QUESTION

More than 15 WHO epidemiological experts have been deployed in Mexico to help authorities combat the outbreaks in the capital Mexico City and two other areas.

So far, 26 swine flu cases in Mexico have been confirmed by laboratory testing, including seven deaths, according to Hartl.

"What we have seen clearly as a result of these epidemiological investigations is there have been probably three smaller waves of human-to-human transmission and now there is an amplification among suspected cases," he said of Mexico.

In all, the WHO has official laboratory confirmation of a total of 73 cases of the new swine flu virus worldwide. They include 40 confirmed cases in the United States, 26 in Mexico, 6 in Canada and 1 in Spain.

The WHO's official total does not include two cases British authorities have confirmed in a Scottish couple returning from their honeymoon in Cancun, Mexico, Hartl said, nor cases which authorities in New Zealand and Israel have announced.

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.

"Has there been co-infection? They may be infected with other illnesses which made their illness more severe. Maybe they are immuno-suppressed. Maybe they didn't get medicines until late, maybe the disease wasn't recognised."

Neither do health authorities know where the virus -- which has elements of avian flu and human influenza -- began.

"We are still looking for the origin of this event," Hartl said.

So far, all suspected and confirmed cases of swine flu in the current outbreak have involved infection from another person, unlike human cases of bird flu that mostly involve contact with infected poultry. (For more Reuters stories on swine flu, click on [nFLU]) (For more Reuters swine flu coverage, please click here:

here

) (For WHO information on swine flu, go to:

here

) (Editing by Jonathan Lynn and Sophie Hares)