The number of swine flu cases worldwide has jumped by at least 24,000 in two weeks to exceed 343,000, while deaths from the A(H1N1) virus edged up to more than 4,100, a US health agency said.
"The World Health Organization (WHO) regions have reported over 343,298 laboratory-confirmed cases of 2009 H1N1 with at least 4,108 deaths, which is an increase of at least 24,373 cases and 191 deaths since September 20," the Centers for Disease Control and Prevention (CDC) said.
The sharp increase in the number of cases was only the tip of the A(H1N1) pandemic iceberg, "as many countries focus surveillance and laboratory testing only on people with severe illness," the CDC said.
The virus, which was first detected in Mexico in April, usually causes illness no more severe than seasonal flu.
Many of the people who have died after contracting the virus had underlying health conditions, and many were also infected with other bacteria, including one which can cause pneumonia.
The swine flu virus continues to be the dominant flu virus in circulation in the world, making up nearly two-thirds of influenza cases reported to the WHO between mid-April and mid-September, the CDC said.
In temperate regions of the southern hemisphere, where influenza season is ending, swine flu was on the wane, the report said.
In parts of the northern hemisphere with temperate climates, flu caused by the A(H1N1) virus was on the increase as influenza season gets under way.
Pharmaceutical firms around the world have been racing to produce vaccines against the virus before the autumn flu season returns in the northern hemisphere.
The United States began rolling out a first batch of flu shots last week.
Tests have shown that one dose of swine flu vaccine can quickly provide sufficient protection in most older children and adults, and that the vaccine is as safe as seasonal flu vaccines, the WHO has said.