Western diplomats have expressed concern about separating children from their parents as part of COVID curbs - a situation that has arisen in Shanghai as the government tries to stamp out the spread of the virus.

The city has been separating COVID-positive children from their parents, citing epidemic prevention requirements, which has prompted a widespread public outcry.

Diplomats from more than 30 countries have written to the Chinese foreign ministry urging authorities not to take such a step.

"We request that under no circumstances should parents and children be separated," said a letter written by the French consulate in Shanghai that was addressed to the foreign affairs office of Shanghai on March 31.

In a separate letter to the Chinese foreign ministry dated the same day, the British embassy in Beijing said it was concerned by "recent instances when local authorities have sought to separate minors who tested positive for COVID-19 from their parents" and requested assurances that this would not happen to diplomatic staff.

The French consulate and British embassy both said they were writing the letters on behalf of other countries, including European Union states as well as others including Norway, Switzerland, Australia and New Zealand after hearing about difficulties caused by Shanghai's lockdown, which the city started carrying out in two stages starting March 28.

Asymptomatic or mild cases should be sent to "a specialized isolation environment with staff who can communicate in English," said the French consulate letter, a copy of which Reuters has seen and verified with two sources.

Currently, asymptomatic cases are sent to centralised quarantine centres, some of which have been described as unsanitary and overcrowded.

The British embassy said there were concerns over the conditions and lack of privacy in recently deployed mobile hospital facilities, adding that isolating in diplomatic housing was a "preferable solution and consistent with our Vienna Convention privileges," in the letter seen by Reuters and verified by two sources.

"The British Consulate-General in Shanghai has been raising its concerns about various aspects of the current COVID policies in relation to all British Nationals in China, with the relevant Chinese authorities," a spokesperson for the consulate said.

The French consulate declined to comment on the letter. The Australian Consulate General in Shanghai, which was cited in the letters, also declined to comment but said it had been engaging with local authorities on the COVID-19 restrictions.

The U.S. did not appear as a signatory on either letter.

However, the U.S. Consul General in Shanghai, Jim Heller, told members of a private chat group for U.S. citizens that the consulate had been underscoring many of the concerns raised by the European letter with the Shanghai government.

A U.S. embassy spokesperson declined to comment on Heller's remarks but said that the way embassy staff were treated in the COVID pandemic was "job one" and that the embassy was engaging on COVID-related policy with the Chinese government.

Other countries, such as Norway, Switzerland and New Zealand, which were mentioned in the letters, did not respond to requests for comment.

The Chinese foreign ministry also did not respond to a request for comment.

On Monday, Shanghai official Wu Qianyu told a news conference that children could be accompanied by their parents if the parents were also infected, but separated if they were not, adding that policies were still being refined.

China has sent the military and thousands of healthcare workers into Shanghai to help carry out COVID-19 tests for all of its 26 million residents as cases continued to rise on Monday amid a city lockdown, in one of the country's biggest-ever public health responses.