U.S. President Joe Biden and Chinese President Xi Jinping held a fifth call as leaders on Thursday, and Xi warned against "playing with fire" over Taiwan, as concerns mounted over a possible visit to Chinese-claimed island by U.S. House Speaker Nancy Pelosi.

Chinese state media said Xi told Biden that the United States should abide by the "one-China principle" and stressed that China firmly opposed Taiwanese independence and interference of external forces.

Sen. Marco Rubio, R-Fla., ripped President Joe Biden's phone call with China's President Xi Jinping.

The call, which was scheduled for Thursday morning, is a "win" for China, Rubio said.

"The phone call itself is a win for Xi, who is facing real political pressure in the lead up to the 20th Party Congress and rising discontent across the country," Rubio said in a statement released by his office Wednesday.

While China’s statements over House Speaker Nancy Pelosi’s possible trip to Taiwan do not stray too far from what it has been saying for decades, insiders in Beijing and Washington worry that they are not empty threats.

“It’s a tough call for the administration,” said Michael Green of the Center for Strategic and International Studies in Washington, who helped shape Asia policy as a senior official in the George W. Bush administration.

Plans for Nancy Pelosi, the US House speaker, to visit Taiwan have prompted opposition from China and the American military but support from Republicans in Washington, including former members of the Trump administration.

Trump’s second secretary of defense, Mark Esper, told CNN: “I think if the speaker wants to go, she should go.”

Mike Pompeo, Trump’s second secretary of state, tweeted: “Nancy, I’ll go with you. I’m banned in China, but not freedom-loving Taiwan. See you there!”

China has ratcheted up warnings to the US about a planned trip to Taiwan in August by House Speaker Nancy Pelosi, even suggesting a possible military response, according to new reports.

The behind-the-scenes rhetoric was significantly stronger than the threats Beijing has made previously regarding US interaction with Taiwan, the Financial Times first reported Sunday, citing six people familiar with the private message to the Biden administration.

The warning signaled a possible military response, the Financial Times said.

Apotential visit to Taiwan by US House Speaker Nancy Pelosi is stirring alarm in President Joe Biden's administration which fears the trip may cross red lines for China -- and that there is no exit path.

China on Monday warned it was "getting ready" for a Pelosi visit to the self-ruling island next month and that the United States would "bear all responsibility for all serious consequences" if she goes.

President Biden stressed Tuesday that his policy toward Taiwan had not changed, one day after forcefully pledging — as he has done before — that the United States would defend the island if it came under assault by neighboring China.

At a meeting here of leaders from the United States, India, Australia and Japan, Biden was asked to elaborate on his unequivocal remarks that were an apparent change to long-standing U.S. policy of deliberately staying unclear about what it would do in such a scenario, a principle known as “strategic ambiguity.”

President Joe Biden insisted Tuesday that the U.S. hasn’t changed its strategic policy on Taiwan, a day after he angered Beijing when he said his administration would be willing to use military force to defend the island.

Biden met with leaders from Japan, India and Australia at their second so-called Quad Leaders’ Summit, which wrapped up Tuesday in Tokyo.

President Joe Biden appearing at odds with his administration for the third time regarding China and Taiwan reverberated around Washington and Beijing.

Despite the White House trying to downplay the differences, Biden intentionally or unintentionally renewed the foreign policy debate over "strategic ambiguity" in the Taiwan Strait at a time when U.S.-China relations are at a nadir and Russia wages war in Ukraine.

President Joe Biden said Monday that the U.S. would intervene militarily if China were to invade Taiwan, saying the burden to protect Taiwan is “even stronger’ after Russia’s invasion of Ukraine. It was one of the most forceful presidential statements in support of self-governing in decades.

Biden, at a news conference in Tokyo, said “yes” when asked if he was willing to get involved militarily to defend Taiwan if China invaded. “That’s the commitment we made,” he added.