WASHINGTON (AFP) — United States President Joe Biden has warned his Chinese counterpart Xi Jinping that violating sanctions on Russia would be a "gigantic mistake."

"I called President Xi — not to threaten at all, just to say to him… that if you think Americans and others will continue to invest in China, based on your violating the sanctions that have been imposed on Russia, I think you're making a gigantic mistake," Biden said in an interview with CBS.

Biden said he delivered the warning in a phone call shortly after Xi met with Putin at the Beijing Winter Olympics on 4 February and expressed support for the Kremlin leader.

But Biden said that so far there is no indication that China has actively supported the Russian war effort with weapons sales.

"Thus far, there's no indication they put forward weapons or other things that Russia has wanted," he said.

Russia invaded its pro-Western neighbor Ukraine on 24 February, resulting in ongoing devastation of cities and towns across much of the country.

Taiwan defense

Meanwhile, Biden said Sunday that US forces would defend Taiwan against a Chinese invasion, while the White House said Washington's policy was unchanged.

Asked by the CBS "60 Minutes" program whether US troops would defend Taiwan, Biden said "yes," if it were "an unprecedented attack."

This was not the first time that Biden has declared US forces would take part in a war between China and Taiwan, with the White House appearing to walk back his comments afterward. The previous time was in May during a visit to Japan.

That time, Biden was also asked whether he'd commit US troops to such a situation and again he said "yes."

"That's the commitment we made," he said.

Washington cut formal diplomatic relations with Taiwan in 1979, switching recognition to Beijing as the sole representative of China. But at the same time, the US maintained a decisive, if delicate role in supporting Taiwan.

Under a law passed by Congress, the US is required to sell Taiwan military supplies to ensure its self-defense against Beijing's vastly larger armed forces.

But Washington has maintained what is officially called "strategic ambiguity" on whether it would intervene militarily.

The policy is designed both to ward off a Chinese invasion and discourage Taiwan from ever provoking Beijing by formally declaring independence.

Asked if the latest statement from Biden signaled a change in that strategic ambiguity, a White House spokesperson said: "The president has said this before, including in Tokyo earlier this year. He also made clear then that our Taiwan policy hasn't changed. That remains true."

Following his Tokyo assertion that "yes," US forces would be involved, Biden was subsequently asked if the strategic ambiguity concept was dead and he replied: "no."

Each time Biden has raised the possibility of US troops fighting to protect Taiwan, China has reacted furiously.

Tensions are already higher than usual in the wake of a rare visit to Taiwan by Nancy Pelosi, a key Biden ally and speaker of the US House of Representatives.

China saw her visit as an escalation and reacted by mounting intimidating sea and air military exercises around Taiwan.

In a recent move by the other chamber of Congress, a US Senate committee last Wednesday took the first step toward changing current policy by seeking to directly allocate $4.5 billion in military assistance over four years for Taiwan, instead of simply continuing to sell arms to the island.