China has urged Mexico to handle the matter "objectively and calmly"
China has denied it is discriminating against Mexican citizens in moves to stem the spread of swine flu.
More than 100 people, including many Mexicans, have been quarantined in mainland China since the virus was found in a Mexican man in Hong Kong.
Mexican officials said their citizens were being unfairly confined. No-one else has displayed any symptoms.
China's foreign ministry said the move was "a purely medical quarantine issue" and not an act of discrimination.
Mexico should "give full understanding to the measures adopted by China and handle this matter objectively and calmly", said a statement on the ministry's website.
The Mexican authorities say they are sending a plane to China to bring back those nationals who wish to return.
Mexico has now confirmed 25 fatalities from the virus, but officials said it appeared that the outbreak could be levelling off.
The a 25-year-old man diagnosed with H1N1 flew from Mexico to Shanghai to Hong Kong.
After the positive test, China decided to track down everyone on his flight and put them into quarantine. It also isolated everyone in the traveller's hotel in Hong Kong.
Mexican officials say more than 70 Mexican citizens in China have been isolated. It is not yet clear whether or not these people had any contact with the infected traveller.
About 50 are being held in quarantine in Shanghai in two five-star hotels, with 10 in Beijing. Mexican nationals are also being held in the city of Wenzhou.
The BBC's James Reynolds in Beijing says China's actions are dramatic but the government may be trying to prove a point to its own people.
In 2003, the Communist Party was widely criticised for covering up the outbreak of the respiratory disease Sars.
So, this time the government may want to show that it is capable of taking early, effective action - even if it leads to a bit of a row with a foreign government, our correspondent says.
There are reports of international flyers being tested for the virus
A spokeswoman at the Mexican Embassy in Beijing said the quarantined Mexicans were being treated well, but added the country's ambassador to China had not been allowed to see those in Beijing's Guomenlu Hotel.
Ambassador Jorge Guajardo said many of them had no connection with the Shanghai-Hong Kong flight which carried the infected Mexican man.
"We are objecting to the fact that they are holding Mexicans in isolation for fear that they might have the flu virus, even though they have no signs of having the flu virus," he said.
On Saturday Foreign Minister Patricia Espinosa criticised China, along with Peru, Argentina, Cuba and Ecuador, for cutting flights to Mexico.
She also urged Mexicans to avoid travel to China, accusing its government of discrimination for the quarantine measures.