GENEVA - H1N1 swine flu is on the rise in China and Japan after triggering an unusually early start to the winter influenza season in Europe, Central Asia and North America, the World Health Organization said on Friday.

According to the U.N. agency's latest official toll, which is thought to underestimate the total spread of the virus, at least 6,071 people worldwide have died as a result of an H1N1 infection since its discovery earlier this year in Mexico and the United States.

Some 359 deaths were recorded in the past week, which saw a big outbreak in Ukraine as well as ongoing spread of the virus across the northern hemisphere.

Intense and persistent influenza transmission continues to be reported in North America without evidence of a peak in activity, the WHO said in its latest update.

In China, after an earlier wave of mixed influenza activity (seasonal H3N2 and pandemic H1N1), pandemic H1N1 influenza activity now predominates and is increasing.

Sharp increases in pandemic flu infections continue to be reported throughout Japan, particularly on the northern island.

China is among more than 20 countries to have launched mass immunization campaigns against the virus.

Most serious illness and deaths have occurred in patients younger than 65, and pregnant women and people with chronic health problems are at high risk.

The WHO's top flu expert Keiji Fukuda said on Thursday that pandemic vaccines given to millions of people had proved to be very safe, providing protection with no unusual side effects.

GlaxoSmithKline and Sanofi-Aventis are among some 25 companies producing pandemic vaccine and other drugmakers including Switzerland's Roche Holding are making antivirals for use as a frontline H1N1 drug.

Some pigs, turkeys and household pets have become infected with the H1N1 flu, but the pandemic virus does not yet appear to be spreading quickly among animals, the WHO said.

(Reporting by Stephanie Nebehay; Editing by Matthew Jones)