A Chinese official has told foreign embassies to stop publishing their own air pollution readings in Beijing.
The US embassy in the Chinese capital regularly warns of serious health effects from smog. The embassy has a monitoring station on its roof and publishes the results hourly on the internet.
The BBC reports air quality in Beijing and other cities is notoriously poor.
Environmental Protection deputy minister Wu Xiaoqing told a briefing in Beijing that only the Chinese government is authorised to publish air pollution data.
Foreign embassies and consulates that did so, contravened the Vienna Convention on Diplomatic Relations and the Vienna Convention on Consular Relations, he said.
The BBC says China has privately demanded that the United States halt its readings in the past, but this is believed to be the first time it has delivered a public warning over the issue.