WASHINGTON - US health officials said they were moving quickly to develop a vaccine against the potentially deadly A(H1N1) flu virus, but cautioned Wednesday that the virus's future behavior remained unpredictable.

Anthony Fauci, head of the Institute of Allergy and Infections Diseases at the National Institutes of Health (NIH), told members of Congress that US scientists have isolated the A(H1N1) virus, which has killed at least 44 people, including two in the United States.

"We have already started the multistep process to develop the vaccine," said Fauci, speaking to a House of Representatives panel on Global Health and Africa.

The next step, currently underway, is developing 'seed viruses' that manufacturers can use to produce pilot vaccines to be tested on humans, Fauci said.

The 2009 A(H1N1) virus "contains genetic pieces from four different virus sources," said Ann Schuchat, a top official at the Centers for Disease Control and Prevention (CDC) who also testified.

"As a result of our investment in pandemic preparedness, we have been able to move within two short weeks to identify a novel virus (and) understand its complete genetic characteristics," said Schuchat, who is also a senior official at the US Department of Health and Human Services.

Whenever US health officials see a new influenza strain they begin to study it in case they need to produce a vaccine to fight it, she said.

"Discussions are underway so that should we need to manufacture a vaccine, we can work towards that goal very quickly," said Schuchat.

The CDC announced Wednesday that the number of confirmed US cases of swine flu surged by 60 percent to 642 from 403, with confirmed infections reported in 41 states.

Two people have died of the flu in the United States: a woman in her thirties suffering from unnamed chronic ailments who died in Texas on Monday, and a Mexican toddler who died in April while visiting Texas.

US officials say there was no cause for alarm, and the Fauci noted that each year some 36,000 people die of seasonal influenza in the United States.

Still, he cautioned that scientists are "only in the earliest stages" of understanding how the H1N1 virus emerged and what impact it may have.

"Viruses are unpredictable, and it is unwise to make predictions about how a virus might behave in the future," he warned.

Fauci warned that drugs currently being used to treat the A(H1N1) virus, such as Tamiflu, may not be useful if the virus mutates.

"Although the virus at this time has caused mostly relatively mild disease in the country," Fauci said, "we do not know whether that might change in the coming months.

"Nor do we know whether the virus will become resistant to the antiviral drugs we have stockpiled," he said.

The virus is contagious and spread from human to human, probably through coughing or sneezing, Schuchat said.

"Some people may become infected by touching something with flu viruses on it and then touching their mouth or nose," she said, adding that there was "no evidence" that the virus "has been found in swine in the United States, and there have been no illnesses attributed to handling or consuming pork."

Also testifying was Dennis Carroll, an advisor to the US Agency for International Development on flu pandemics, who said that scientists had yet to determine the virulence of the virus. "We do not know whether this outbreak will become severe or remain mild," he said.

However, acting CDC head Richard Besser said separately at a news briefing that he expects the swine flu outbreak to reach a pandemic level.

"With the number of cases in other countries, I would be surprised if we don't get to level six" on the World Health Organization's (WHO) six-phase pandemic alert scale, Besser said.

Level five is characterized by human-to-human spread of the virus into at least two countries in a single region, and means a pandemic is imminent. Level six means a pandemic is under way.

Mexico is at the heart of the outbreak of the new strain of A(H1N1) or swine flu, and has also been the country the hardest hit by the virus, with 1,070 confirmed cases and 42 deaths.