Two and a half months ahead of the Beijing Olympics, China has been struggling to clear a hurdle that could thwart President Xi Jinping's efforts to host the Winter Games in a complete form -- the safety of female tennis star Peng Shuai.

On Sunday, the International Olympic Committee said its president Thomas Bach held a 30-minute video call with Peng, but the move has failed to eliminate concerns by the United States and other democratic nations that she has been detained by Chinese authorities.

File photo taken Oct. 4, 2017, shows Chinese tennis player Peng Shuai at the WTA Tour's China Open in Beijing. (Kyodo) ==Kyodo

Fears about Peng's safety mounted internationally after a social media post under her name spread online in early November, saying she had had sexual relations with former Vice Premier Zhang Gaoli when he was a top party official in Tianjin.

Should the issue not be resolved before the Beijing Olympics, the United States would implement a "diplomatic boycott" of the event slated for February, as President Joe Biden has been criticizing China's alleged human rights abuses, pundits said.

The development of this matter surrounding Peng is also expected to force Japan, China's neighbor that hosted the Tokyo Olympics this summer, to contemplate whether it should follow the United States, its close security ally, they said.

While China has been trying to "make a story" that Peng is safe and free at home, "nobody can believe it" unless she "speaks the truth in public," a diplomatic source said.

If this situation continues, the Biden administration is "sure to adopt a tougher approach" against Beijing, and Japan, whose economy has heavily relied on China, would become unable to "take an ambiguous response," the source added.

Peng has won the women's doubles at Wimbledon and the French Open -- two of four major global tournaments. Zhang served as a member of the Politburo of the ruling Communist Party's Central Committee from 2012 through 2017 under Xi's leadership.

On Nov. 2, the post on Weibo, China's equivalent to Twitter, in which Peng confessed an unhealthy relationship with Zhang, suddenly appeared but was swiftly deleted.

Afterward, speculation was rife that Peng was missing and detained, prompting many famous tennis players, including current world No. 1 Novak Djokovic of Serbia and Japan's Naomi Osaka, to express anxiety about Peng's safety.

Late last week, the state-run China Global Television Network tweeted an image of what it said was an email from Peng to Steve Simon, the head of Women's Tennis Association.

The email said in English, "I'm not missing, nor am I unsafe. I've just been resting at home and everything is fine." Doubt, however, lingered over whether it was really written by her.

In a presumed attempt by the Chinese government to tamp down the controversy regarding Peng, photos showing her smiling and waving her hand at a tennis tournament in Beijing were released by the event's organizer on Sunday.

Hu Xijin, editor-in-chief of the Communist Party's Global Times newspaper, said in a Twitter post in English, "Can any girl fake such sunny smile under pressure? Those who suspect Peng Shuai is under duress, how dark they must be inside."

The IOC, which some analysts say hopes for financial contribution from China, announced on the same day that Peng talked with Bach online. Later, the organization unveiled a still image from the chat, but video of the meeting has not been presented to the public.

On Monday, the IOC was accused by New York-based Human Rights Watch of promoting Chinese state propaganda, while U.S. State Department spokesman Ned Price told a press conference that Washington is "closely monitoring" developments about Peng.

Supplied photo shows International Olympic Committee President Thomas Bach (back to camera) talking with Chinese tennis player Peng Shuai (on screen). (Copyright IOC/Greg Martin)(Kyodo)

Price's remarks came as Biden acknowledged last week that he is considering a diplomatic boycott of the Beijing Olympics, suggesting that no U.S. government representatives will attend the global sporting event.

The boycott, which would not affect the participation of U.S. athletes, is seen as a response to China's alleged human rights violations against Uyghur Muslims in its far-western Xinjiang region that Washington has labeled as "genocide."

Given that Biden has put emphasis on human rights issues, he cannot overlook "Peng's official silence," a source familiar with Sino-U.S. relations said, adding Washington is "highly likely" to carry out a diplomatic boycott of the Beijing Olympics.

If that happens, Japan will be "stuck between a rock and a hard place," said Stephen Nagy, a senior associate professor at International Christian University in Tokyo.

"Japan's position will be nuanced to ensure China saves faces but not big enough to create problems at the economic level," he said. "In the case that Tokyo does implement a broad diplomatic boycott at all levels, Beijing and Xi would certainly lose face."

China would "retaliate by upping pressure on Japan" in the East China Sea or "pressuring Japanese businesses domestically through increased inspections, regulation enforcement or interfering with critical supply chains that impact Japan," Nagy said.

Tai Wan-chin, a professor emeritus at Tamkang University in New Taipei City, also said if the United States calls for a diplomatic boycott, democratic counties are "very likely to follow suit."

Even though the Japanese government under Prime Minister Fumio Kishida may want to "retain flexibility in its policy" toward China, it "will not eventually resist U.S. persuasion or pressure," he said.

In November, Kishida tapped Yoshimasa Hayashi, a former defense minister who heads a cross-party lawmakers' group that deepens Japan-China ties, as foreign minister, cementing the view that Tokyo will pursue a well-balanced diplomatic strategy toward Beijing.

As for Peng, Hayashi said Wednesday that Japan is keeping an eye on the situation and "strongly hopes concerns will be dispelled as soon as possible," while noting that nothing has been decided at this point on a diplomatic boycott of the Beijing Olympics.

Kishida, meanwhile, set up a new post of special adviser to the prime minister on human rights, in an apparent bid to tackle China's alleged repression in Xinjiang and its crackdown on Hong Kong's pro-democracy movement.