TAIPEI (Reuters) - Taiwan faces a more complicated task this year in seeking to attend a U.N. health agency meeting than it has in the past, a senior official said on Tuesday, as the self-ruled island braces for another setback in ties with rival China.

Diplomatically isolated Taiwan is not a member of the United Nations, which recognizes “one China” centered on Beijing. China, in turn, views democratic Taiwan as a renegade province to be taken back with the use of force, if necessary.

The decision-making body of the World Health Organization (WHO) will hold its annual meeting from May 22 to 31 in Geneva, but Beijing warned last year that acceptance of the “one China” principle was a condition for Taiwan’s attendance.

Taiwan has attended the meeting of the body, known as the World Health Assembly (WHA), as an observer since 2009, under the China-friendly predecessor of Taiwan President Tsai Ing-wen. Invitations usually go out early in May, and Taiwan received one last year before Tsai formally took office in mid-May.

“Our efforts this time have not been as smooth as before,” Taiwan foreign ministry spokeswoman Eleanor Wang told a regular news briefing.

Taiwan has been lobbying the WHA to attend the meeting and has roped in its friends, including the United States, its few diplomatic allies and non-government bodies to help, she said.

The factors influencing the situation were “more complicated” this year, Wang added, but made no direct reference to China.

In Beijing, when asked about Taiwan and the WHA meeting, Chinese Foreign Ministry spokesman Geng Shuang said, “On issues involving Taiwan in the international sphere, we consistently advocate handling the issue according to the one China principle.”

China distrusts Tsai and her ruling party, which traditionally advocates independence for Taiwan. Soon after Tsai took power, Beijing cut official communication channels with her government to try to pressure her to concede that Taiwan is part of China.

Taiwan was snubbed last year by U.N. aviation agency, the International Civil Aviation Organization (ICAO), which cited the “one China” policy for not inviting it to a meeting in Canada.

China has claimed sovereignty over Taiwan since 1949, when Mao Zedong’s Communist forces won the Chinese civil war and Chiang Kai-shek’s Nationalists fled to the island.