A notice dated May 3 on the Web site for the Chinese Embassy and its consulates in the U.S. said that all visa applications would now require six business days to process, with express and rush services for visa applications suspended until further notice.
It is unclear exactly why the rules have been changed, but it came hot on the heels of strident measures by China to contain any possible spread of swine flu, including quarantining of some foreign nationals.
The American Embassy in Beijing said four U.S. citizens are now or have been quarantined in China due to swine flu fears.
Embassy spokeswoman Susan Stevenson said Tuesday two of the Americans were in Beijing while the other pair was in the southern province of Guangdong.
Stevenson declined to go into specifics of the individual cases, citing privacy reasons, but said only two of them remained in quarantine.
She said they were either placed under quarantine because of flu-like symptoms or because they were close to "cases of interest to the Chinese authorities."
The new visa regulation, effective as of May 4, appears to apply to all Chinese visas, including tourist and business categories. Visa applicants are also required to fill out a form declaring which countries and U.S. states they had visited two weeks prior.
Previously, U.S. nationals could obtain visas in as little as one day.
More than 1,400 people globally have been infected with swine flu, with Mexico reporting the most confirmed cases with 802. The United States so far has reported 380 cases in more than 30 states.
On Tuesday, Foreign Ministry spokesman Ma Zhaoxu refused to address the specific visa changes for Americans, saying only that "relevant adjustment (to the visa policy) is non-discriminatory and is not targeted at any country. The adjustment of visa policy will not affect the normal entry of foreigners and exchanges of people."
The new rules do not appear to be in effect for any other country, including Spain or Canada, where swine flu has also been detected.
China has already earned the ire of the Mexican government for its aggressive quarantine measures after a Mexican traveler flying to Hong Kong via Shanghai was diagnosed with the illness over the weekend.
More than 70 Mexicans were quarantined in hotels and hospitals in mainland China. A plane chartered by the Mexican government arrived in several cities in China on Tuesday to pick up these and other Mexican citizens and take them home.
Mexican officials have accused Beijing of treating their citizens unfairly, but China has denied singling out Mexicans, saying it was purely a medical matter and that it hoped Mexico would be "objective and calm."
A group of 29 Canadian students and their professor were also being held in isolation in China.
Last year, China severely tightened visa regulations ahead of the Olympic Games in August as part of a wider security clampdown, and earlier this month, travel agencies in Hong Kong reported that visa restrictions were being tightened again ahead of the 60th anniversary in October of the communist nation's founding.
Last week, government spokeswoman Jiang Yu said the reports of changes to visa regulations were "groundless."
Meanwhile, Mexico on Mondaypreparing to reopen businesses and schools even as the virus sickened more than 1,400 people in 20 countries.
World health officials said the global epidemic is still in its early stages, and that a pandemic could be declared in the days to come. But Mexico's president said it was waning at its epicenter, justifying Wednesday's end to a five-day nationwide shutdown he credits for reducing the spread of the new virus.
While Mexico slowly reopens for business, American officials remain hopeful (but cautious) that the worst is over.
Homeland Security Secretary Janet Napolitano said, "We have started to see encouraging signs that this virus may be mild and that its spread may be limited.
"We realize that this is not the time to rest; we are preparing fully," she said.
St. Francis Preparatory School in New York opened its doors yesterday, but more than five hundred schools across the country are still closed, leaving 330,000 healthy children sitting at home with an unscheduled vacation.
Although officials agree the strain looks very much like seasonal flu, they warn it could roar back during the fall flu season, reports CBS News correspondent Bianca Solorzano.
CBS Early Show medical correspondent Dr. Jennifer Ashton said, "Any time we've had a new virus - and it's already demonstrated that it's changed or mutated, it's adapted in its new host - there is always a chance when it comes back in subsequent or following flu seasons, it could be even stronger."
2009 H1N1 Flu Outbreak Map:
This is a map depicting confirmed and suspected cases of the 2009 H1N1 outbreak, with contributors from all over the world, from a variety of backgrounds including health, journalism, technology.