China has demanded that foreign firms, and airlines in particular, begin referring to Taiwan as Chinese territory on their websites, along with Hong Kong and Macau, a move described by the White House in May as "Orwellian nonsense".

Numerous non-U.S. carriers, such as Air Canada, Lufthansa and British Airways have already made changes to their websites, according to Reuters checks.

But several U.S. companies, including Delta Air Lines and United Airlines, were among carriers that sought extensions to a May 25 deadline to make the changes. The final deadline is July 25.

Speaking at a daily news briefing, Chinese Foreign Ministry spokesman Lu Kang said it was the broad consensus in the international community that there was only one China and that Taiwan was part of it, and that this was not up for negotiation.

Foreign companies operating in China must respect the country's sovereignty and territorial integrity as well as the feelings of the Chinese people, he added.

"I again stress, the 'one China' principle is the political basis of Sino-U.S. ties, and brooks no negotiations or consultations," Lu said.

"The U.S. government should urge the relevant companies to scrupulously abide by the one China principle and rectify their websites as soon as possible."

China has rejected U.S. requests for talks over how American airlines and their websites refer to Taiwan, according to sources, adding to tensions in a relationship already frayed by a major trade dispute.

The United States switched diplomatic recognition from Taiwan to China in 1979, acknowledging that China takes the position that there is one China and Taiwan is part of it. But the United States is also Taiwan's biggest ally and arms supplier.

Taiwan is China's most sensitive territorial issue. Beijing considers the self-ruled, democratic island a wayward province. Hong Kong, a British colony for more than 150 years until 1997, and neighbouring Macau, administered by Portugal for centuries until 1999, are part of China but are run largely autonomously.

(Reporting by Ben Blanchard; Editing by Nick Macfie)