At least four US citizens are now or have been quarantined in China under strict measures put in place to curb the spread of swine flu, a US embassy official said on Tuesday.
"We are aware of four individuals who have been or are currently in quarantine," embassy spokeswoman Susan Stevenson told AFP.
"I believe two are in quarantine now and two were in quarantine."
China has been sharply criticised for implementing stringent measures aimed at preventing the spread of the influenza A(H1N1) virus, with Mexico calling the policy discriminatory and Canada questioning the isolation of 22 students.
On Tuesday, Mexico was preparing to fly home dozens of nationals quarantined against their will in Beijing.
Stevenson said the US government has not issued any statements of concerns over China's treatment of the Americans.
"The cases differ, they could have been quarantined due to their proximity to someone who may have been sick, or they could have displayed symptoms and placed in quarantine as a precaution," Stevenson said.
Two of the US citizens were quarantined in southern Guangdong province and two were quarantined in Beijing, she said, declining further comment.