Beijing, China - Chinese President Xi Jinping called for advancing efforts towards "reunification" with Taiwan, the self-ruled island Beijing claims as its own, in a speech on Wednesday on the eve of China's national day.

Xi rejected "external interference" over Taiwan in his address ahead of the 77th anniversary of the founding of the People's Republic of China, days after he told US President Donald Trump that Washington should oppose the island's independence.

Beijing claims democratic Taiwan as part of its territory and has not ruled out using force to take it.

"We must deepen cross-Strait exchanges and cooperation, resolutely crack down on 'Taiwan independence' separatist forces, oppose interference from external forces, promote the peaceful development of cross-Strait relations, and advance the great cause of national reunification," Xi said.

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His address was made to hundreds of guests at Beijing's Great Hall of the People, according to state news agency Xinhua.

The People's Republic of China was founded on October 1, 1949, after the Communist army defeated nationalist Kuomintang forces at the end of a civil war lasting two decades.

The defeated nationalists fled to Taiwan.

The island relies on the US for its security, including through arms purchases.

Xi made a state visit to Washington last week, during which he told Trump to show "prudence" over Taiwan and urged the US to oppose the island's independence.

Washington had already paused a $14 billion weapons purchase by Taiwan in May, with Trump calling it a "very good negotiating chip" in dealings with China, which raised fears that Taiwan's security could be traded away.

Xi also warned in his speech that China faced "complex international and domestic situations" that demanded "accelerating the building of a new pattern of development".

"We must adhere to an independent foreign policy of peace... to inject more certainty and positive energy into a world intertwined with changes and turmoil," he said, according to Xinhua.