The death toll from swine flu topped 50 as Costa Rica reported its first fatality from the virus and Japan and Norway joined a growing list of nations with confirmed cases.
In Costa Rica, Health Minister Maria Luisa Avila said the victim was a 53-year-old man who died on Saturday after more than a week in a hospital in the capital San Jose.
The man had been suffering from other health problems, including diabetes and a chronic pulmonary condition. Eight other Costa Ricans had contracted A(H1N1) influenza, as the virus is officially known, Avila said.
In Tokyo, a student who recently returned from North America has tested positive for swine flu in Japan's fourth confirmed case of the virus, a government official said.
The Japanese teenager was among 49 passengers who were quarantined near Tokyo's Narita International Airport following confirmation of Japan's first three cases of the A(H1N1) influenza virus Saturday.
Earlier, the health ministry said a teacher and two of the student's schoolmates, who had been on a high school trip to Canada, tested positive for the virus.
The other passengers, whose nationalities were not released by authorities, arrived in Tokyo from the US city of Detroit Friday on board the school party's flight.
"The four infected people have received medical treatment in hospital but we have heard that they are not in extremely bad condition," said Yutaka Ohno, a health ministry spokesman.
In Norway, health officials announced the first two confirmed swine flu cases, both young adults who had recently returned from Mexico.
There have now been 48 deaths recorded in Mexico along with two US deaths and one each in Canada and Costa Rica. The World Health Organization has still to confirm three of the Mexican fatalities and the latest one from Costa Rica.
While Mexico was the epicenter of the global swine flu epidemic, the United States has overtaken its southern neighbor to become the country with the most number of cases.
The United States now has 2,254 confirmed cases with 104 people hospitalized; Mexico has recorded 1,626 cases including its 48 deaths, and done 5,580 tests.
US cases were confirmed in 44 of the 50 US states and the capital Washington. Health authorities were now focusing on the characteristics of the new virus and on developing a vaccine, said Anne Schuchat of the US Centers for Disease Control and Prevention (CDC).
Swine flu has now been reported in a total of 31 nations and the latest WHO figures, which don't include new cases in Costa Rica, Japan, Mexico, Norway, Spain and the United States, put the total number of infected at 3,440.
Canada has the world's third highest number of swine flu infections, with 281 cases, and on Friday reported its first death.
The sharp increase in cases from WHO's Friday count of 2,500 cases reflected the doubling of confirmed cases in the United States.
Japanese Prime Minister Taro Aso said the three confirmed cases were not considered a domestic outbreak because they were intercepted at the airport.
The teacher and students had stayed in Oakville, west of Toronto, on a school trip from April 24 until last Thursday. The teacher remained sick while the two students were recovering, officials said.
Mexico gradually returned to life this week, reopening tourist sites, Mexico City eateries, theaters and cinemas after a shutdown of more than a week. Universities and high schools reopened on Thursday and primary schools were due back on Monday.
US President Barack Obama warned the United States on Friday that it was not out of the woods yet, as across the nation's northern border a woman in western Alberta province became the first person in Canada to succumb to the disease.
"We are seeing that the virus may not have been as virulent as we at first feared," Obama said at a Spanish-language town hall-style meeting at the White House.
"But we are not out of the woods yet. We still have to take precautions."
The US president warned the autumn and winter flu season later in the year could be "even worse" and see cases spike again.