WASHINGTON - The number of confirmed cases of swine flu in the United States continued to rise Monday with US health authorities saying 286 people in 36 states were now infected with the virus.

Confirmed infections have risen by more than a quarter since the previous tally of 226 in 30 states, released Sunday.

'We continue to see spread widely across the United States,' said acting head of the US Centres for Disease Control and Prevention, Richard Besser, adding that there were more than 700 probable cases in 44 states.

'A probable case is someone who has flu-like symptoms and is tested with a regular flu test... We're seeing over 99 percent of those are ending up confirmed,' Besser said.

The median age of confirmed cases is 16 years, with the youngest sufferer aged just three months and the oldest 81 years.

Nearly two-thirds of the confirmed cases were in people under the age of 18.

Thirty-five people have been hospitalized in the United States, but Besser had no details about them, the severity of their illness or whether they had any other underlying medical conditions.

There remains only one death in the United States so far, a Mexican toddler who was visiting relatives in Texas, Besser said.

Although there have been encouraging signs, with the virus apparently levelling off in Mexico, its suspected epicenter, and the illness not very severe in the United States, Besser said the H1N1 flu was expected to spread to all 50 states.

World Health Organization (WHO) officials estimated Monday that there are 1,003 cases of swine flu spread through 20 countries.