Beijing warned the United States on Monday against interfering in Hong Kong's affairs after US Vice President Joe Biden met with two of the city's outspoken pro-democracy campaigners last week.

In an unusually high sign of support, Biden attended talks at the White House on Friday with Martin Lee, a founder of Hong Kong's opposition Democratic Party, and Anson Chan, former number two in the city's government.

Biden "underscored our long-standing support for democracy in Hong Kong", the White House said in a statement.

In the first official response from Beijing, Chinese state-run news agency Xinhua quoted an official as saying that the US should "proceed discreetly" to prevent damaging China-US relations.

Hong Kong -- a former British colony which is now a special administrative region of China -- is at a critical time of political reform and Beijing opposes interference by any country in China's internal affairs, a foreign ministry spokesman in Hong Kong said, according to Xinhua's dispatch.

Lee and Chan used their visit to Washington last week to speak out against what they described as growing interference by Beijing in Hong Kong.

Britain returned the financial hub to China in 1997 under a deal that granted it semi-autonomous status and enshrined civil liberties not guaranteed in mainland China.

China has promised direct elections for Hong Kong's chief executive in 2017, but many pro-democrats fear that Beijing's Communist rulers will control the choice of candidates to secure the election of a sympathetic official.

Beijing has ruled out demands that voters be allowed to choose which candidates can stand for the top position, an issue which regularly sparks angry protests.

China has previously hit out at pledges by other foreign officials to support Hong Kong's pro-democracy activists.

In September, China described comments by British Foreign Office minister Hugo Swire that Britain was "ready to support in any way we can" those pressing for greater democracy in Hong Kong as "irresponsible".