China has tightened visa rules for citizens from the United States, which has reported the second highest number of A (H1N1) flu cases in the world.

A notice dated May 3 on the website for the Chinese Embassy and its consulates in the US 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 amid a global efforts to contain the spreading A (H1N1) flu, formerly known as swine flu.

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 US states they had visited two weeks prior.

Previously, US nationals could obtain visas in as little as one day.