BEIJING (Reuters) - Relations between China and Taiwan this year will be “even more complex and grim” and Beijing will resolutely oppose any form of separatist activity, a senior Chinese official said, but added China would also maintain peace and stability.

Yu Zhengsheng, the ruling Communist Party’s fourth ranked leader, was speaking at an annual work conference on policy towards the self-ruled island. His comments came in the wake of concerns that, under a new presidency, the United States is shifting its stance toward Taiwan.

U.S. President Donald Trump, who was inaugurated on Friday, broke with decades of precedent last month by taking a congratulatory telephone call from Taiwan President Tsai Ing-wen.

That, along with subsequent comments by Trump that the “one China” policy was up for negotiation, has infuriated Beijing, which views Taiwan as a wayward province, to be brought under its control by force if necessary.

Proudly democratic Taiwan has shown no interest in being run by China.

China is deeply suspicious of Tsai, whose ruling Democratic Progressive Party espouses the island’s formal independence, a red line for Beijing, and has cut off a formal dialogue mechanism with the island.

China must “resolutely oppose and check any form of ‘Taiwan independence’ separatist activity, maintain national sovereignty and territorial integrity, and maintain the peaceful development of cross-Taiwan Strait relations and peace and stability”, state-run Xinhua news agency paraphrased Yu as saying.

In a letter to Pope Francis released by her office on Friday, Tsai said Taiwan aspires to create a “new era” of peace with China as military action cannot resolve problems.

Defeated Nationalist forces fled from China to Taiwan in 1949 after losing a civil war with the Communists.

The report made no mention of Tsai’s letter to the Pope. The Vatican is one of a handful of countries to maintain formal ties with Taiwan.

Yu said that China would seek to “unite” all parties and groups in Taiwan that accept both sides of the Taiwan Strait are part of “one China”.

China will also look at ways of making life easier for Taiwanese people to invest, work and live in China, Yu added, without giving details.

Since ties began thawing in the 1980s, Taiwanese have invested billions in China, attracted by a common culture and language and the lure of China’s vast market and cheap pool of labor.