WASHINGTON (AFP) - The United States is in the grip of a deadly influenza outbreak that has hit harder and earlier than in previous years, and has claimed the lives of at least 18 children.

"It looks like the worst year we have had since 2003-2004," said Dr Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.

He said this year's influenza strain, which has sickened thousands across the country, is particularly severe.

"The type of flu is one that generally is more serious. It's the H3N2 variety, which is historically more serious than we see with other types of virus," he said.

The epidemic, which broke out at the beginning of December, has caused some 2,200 hospitalisations across the United States, federal health officials said.

Particularly hard hit has been the north-eastern city of Boston, where officials have declared a public health emergency.

City officials there said there have been about 700 confirmed cases of flu so far, almost 10 times the number from this time last year.

"This is the worst flu season we've seen since 2009, and people should take the threat of flu seriously," Boston Mayor Thomas Menino said in a statement.

"I'm urging residents to get vaccinated if they haven't already. It's the best thing you can do to protect yourself and your family. If you're sick, please stay home from work or school," he said.

Dr Joe Bresee, chief of the Epidemiology and Prevention Branch of the Centre for Disease Control and Prevention's Influenza division, said officials do not yet know how much worse this year's outbreak will get.

"While we can't say for certain how severe this season will be, we can say that a lot of people are getting sick with influenza and we are getting reports of severe illness and hospitalisations," he said.

US states, particularly in the north-east of the country, have seen a sharp spike in emergency room visits from patients reporting flu-like symptoms.

In Allentown, Pennsylvania, one hospital had to erect a large outdoor tent to admit and treat the large number of flu sufferers.

Health officials said that the flu vaccine is a good match for the strain of influenza circulating around the nation, and confers about 60 per cent to 65 per cent protection against the illness.

"You might get the flu but it will likely be less severe if you are vaccinated," Dr Fauci said.