The level of air pollution in China's heaving capital varies, depending on the wind, but a cocktail of smokestack emissions, vehicle exhaust, dust and aerosols often blankets the city in a pungent, beige shroud for days on end.

Many residents dismiss the common official readings of "slight" pollution in Beijing as grossly under-stated.

The U.S. embassy has installed a monitoring point on its roof which releases hourly air-quality data via a widely followed Twitter feed. The U.S. consulates in Shanghai and the southern city of Guangzhou provide a similar service.

While China tightened air pollution monitoring standards in January, the official reading and the U.S. embassy reading can often be far apart.

Chinese experts have criticised the single U.S. embassy monitoring point as "unscientific".

Deputy Environment Minister Wu Xiaoqing went a step further, saying such readings were illegal and should stop, though he did not directly name the United States.

"According to the Vienna Convention on Diplomatic Relations ... foreign diplomats are required to respect and follow local laws and cannot interfere in internal affairs," Wu told a news conference.

"China's air quality monitoring and information release involve the public interest and are up to the government. Foreign consulates in China taking it on themselves to monitor air quality and release the information online not only goes against the spirit of the Vienna Convention ... it also contravenes relevant environmental protection rules."

Chinese Foreign Ministry spokesman Liu Weimin called on foreign diplomatic missions to respect China's laws and regulations and to stop issuing the readings, "especially over the Internet".

"If the foreign embassies want to collect this kind of information for their own staff and diplomats, I think it's no problem," Liu told reporters. "They can't release this information to the outside world."

The U.S. embassy acknowledges on its website (beijing.usembassy-china.org.cn) that its equipment cannot be relied upon for general monitoring, saying "citywide analysis cannot be done ... on data from a lone machine".

Richard Buangan, the spokesman for the U.S. embassy in Beijing, said the air quality monitor in Shanghai was "an unofficial resource for the health of the consulate community".

That was consistent with the Beijing embassy's "practice of making air quality data available to the American community in Beijing", Buangan said in emailed comments to Reuters.

Wu said China's air quality standards were drawn up in consultation with the World Health Organization and "accorded with the present situation in our country".

"What needs saving is the country's air quality, not the government's face," Zhou Rong, an energy campaigner for Greenpeace, said in emailed comments. "The environmental authorities must stop finger pointing and start taking actions that really address the issue."

Despite his criticism, Wu acknowledged that China's air quality and overall environmental situation remained precarious, with more than one tenth of monitored rivers rated severely polluted, for example.

Wu said the government was studying a long-mooted environment tax on polluting industries, though he did not give a timetable for when it might come into effect.

(Additional reporting by Sui-Lee Wee; Editing by Nick Macfie)

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