WASHINGTON — US House of Representatives Speaker Nancy Pelosi's rumored plan for a trip to Taiwan has infuriated China and left the White House with a serious geopolitical headache. How big a problem is this?

On Monday, China warned of "serious consequences" if Nancy Pelosi were to proceed with a visit to Taiwan in the coming weeks.

Second in line to the presidency, Pelosi would be the highest ranking US politician to travel to the self-governing island democracy since 1997.

However, China sees self-ruled Taiwan as a breakaway province that will eventually be part of the country again - and has not ruled out the possible use of force to achieve this.

The potential trip has not only rankled Beijing - the Biden administration has reportedly tried to dissuade the California Democrat from going.

Last week, President Joe Biden told reporters "the military thinks it's not a good idea", but his White House has called Chinese rhetoric against any such trip "clearly unhelpful and not necessary".

The state department says Pelosi has not announced any travel and the US approach to Taiwan remains unchanged.

There is strong bipartisan support for Taiwan among the American public and in the US Congress.

And over a congressional career spanning 35 years, Speaker Pelosi has been a vocal critic of China.

She has denounced its human rights record, met with pro-democracy dissidents, and also visited Tiananmen Square to commemorate victims of the 1989 massacre.

Pelosi's original plan was to visit Taiwan in April, but it was postponed after she tested positive for Covid-19.

She has declined to discuss details of the trip, but said last week that it was "important for us to show support for Taiwan".

Beijing views Taiwan as its territory, and has repeatedly raised the spectre of annexing it by force if necessary.

Chinese officials have expressed anger over what they view as growing diplomatic engagement between Taipei and Washington. This includes a surprise visit to the island by six US lawmakers in April.

On Monday, Chinese foreign ministry spokesman Zhao Lijian warned his country would take "firm and resolute measures" if Mrs Pelosi went ahead with her visit.

"And the US will be responsible for all of the serious consequences," he said.

A spokesman of the Chinese ministry of defence seemed to suggest there could even be a military response.

"If the US side insists on going ahead, the Chinese military will never sit idle and will take strong measures to thwart any external interference and separatist attempts for 'Taiwan independence'," Colonel Tan Kefei told China Daily.

At its party congress later this year, the Chinese Communist Party is set to re-elect Xi Jinping to an unprecedented third term as president.

President Biden - who last spoke with President Xi in March - has said they will speak over the phone again in the next few days, on a range of topics including Taiwan and other "issues of tension".

The call comes as US officials warn of a Chinese military build-up in the Asia-Pacific region and "aggressive and irresponsible behaviour" in the South China Sea.

The threats of retaliation over Mrs Pelosi's visit have raised concerns over China's possible response.

When then-US Health Secretary Alex Azar flew to Taiwan in 2020, Chinese air force jets crossed over the mid-line of the Taiwan Strait - the narrow waterway between the island and its giant neighbour - within range of Taipei's missiles.

Last week, the former editor of China's state-run Global Times newspaper suggested a "shocking military response" may be in store for Pelosi.

"If Pelosi visits Taiwan, [People's Liberation Army] military aircraft will accompany Pelosi's plane to enter the island, making a historic crossing of the island by military aircraft from mainland for the 1st time," Hu Xijin wrote.

Until now, Washington's policy of "strategic ambiguity" has meant the US has been deliberately unclear about whether or how it would defend Taiwan in the event of a large scale attack on the island. — BBC