TAIPEI (Reuters) - China was stepping up a campaign to exert influence over Taiwan, including its upcoming presidential election, a senior U.S. official said on Thursday, at a time of heightened tension between the self-ruled island and Beijing.

China has increased military and diplomatic pressure on Taiwan, whose president, Tsai Ing-wen, Beijing suspects of pushing for the island's formal independence, a red line for China which has never renounced the use of force to bring Taiwan under its control.

The island is gearing up for a presidential election in January that could shake up the political landscape, with contenders including Terry Gou, chairman of Apple supplier Foxconn.

"They've obviously stepped up campaigns of disinformation and direct influence against Taiwan," James Moriarty, chairman of the American Institute in Taiwan, told Reuters.

Moriarty, the most senior U.S. official in charge of Taiwan relations, said Beijing's attempts to influence Taiwan were a concern for the United States.

"I do worry greatly about attempts to influence Taiwan's democratic processes and I believe many Taiwanese share that concern," he said.

Tsai, whose pro-independence Democratic Progressive Party suffered a major poll defeat in November, warned last year that Beijing was exerting pressure on the island to influence its politics.

The United States has no formal ties with Taiwan but is bound by law to help provide the island with the means to defend itself and is its main source of arms.

Beijing regards Taiwan as its sacred territory and regularly calls it the most sensitive and important issue in ties with the United States.

Chinese bombers and warships again conducted drills around Taiwan on Monday. The United States denounced the exercises as "coercion" and a threat to stability in the region.

(Reporting by Yimou Lee; Editing by Robert Birsel)