Beijing wants to seize Taiwan "on a much faster timeline" than previously considered, US Secretary of State Antony Blinken said Monday, warning that President Xi Jinping was leading China in a more aggressive direction.

Xi is on the cusp of securing a third five-year term at the helm of the world's most populous nation, delivering a landmark Communist Party Congress speech on Sunday that hailed his decade in power and restated his vow to one day "reunify," or forcefully take Taiwan.

"We've seen a very different China emerge in recent years under Xi Jinping's leadership," Blinken told a forum at Stanford University with former secretary of state Condoleezza Rice.

"It is more repressive at home; it's more aggressive abroad. And in many instances that poses a challenge to our own interests as well as to our own values," he added.

Blinken accused Xi of "creating tremendous tension" by changing the approach toward self-ruled Taiwan, which China's Communist Party has never controlled but claims as its own.

He said China had made a "fundamental decision that the status quo was no longer acceptable and that Beijing was determined to pursue reunification on a much faster timeline," though he gave no hard estimate or date.

Senior US military figures have previously sounded the alarm that China has expanded its military forces to the point where it could soon have the capability to pull off an invasion of Taiwan.

China's stance has long been that it seeks "peaceful reunification" with Taiwan but reserves the right to use force if necessary, especially if the island formally declares independence.

But the rhetoric and actions towards Taiwan have become more pronounced under Xi, China's most assertive leader in a generation.

He has tied taking Taiwan to his vision of the "great rejuvenation of the Chinese nation" and has previously said the goal of reunification cannot continue to be passed indefinitely from generation to generation.

In Sunday's speech, he repeated similar themes, saying the "wheels of history are rolling on towards China's reunification" and that "we reserve the option of taking all measures necessary."