Secretary of State Antony Blinken leveled the allegations as US-Chinese relations have been further tested by Washington's shooting down this month of what it said was a large Chinese spy balloon.

The European Union also sounded the alarm over munitions in the Ukraine conflict -- saying that severe ammunition shortages facing Ukrainian forces had to be overcome within weeks.

Blinken told CBS that China was now "considering providing lethal support" to Moscow ranging "from ammunition to the weapons themselves."

"We've made very clear to them that that would cause a serious problem for us and in our relationship," he added.

He made similar comments in a series of interviews from Germany, where on Saturday he attended the Munich Security Conference and met with his Chinese counterpart, Wang Yi.

Also at the Munich conference, EU foreign policy chief Josep Borrell issued a stark warning about Ukraine's dwindling supplies of bullets and similar munitions as it fights back against Russia's invasion.

"(Let's) accelerate our military support to Ukraine because Ukraine is in a critical situation from the point of view with ammunition available," Borrell said.

"This shortage of ammunition has to resolve quickly, it's a matter of weeks."

There have been concerns China is deepening ties with Russia despite the conflict -- but Wang insisted that Beijing was playing a constructive role, and would support dialogue and potential peace talks.

Appearing Sunday on ABC, Blinken emphasized that US President Joe Biden had warned his Chinese counterpart, Xi Jinping, as long ago as last March against sending weapons to Russia.

Since that time, "China has been careful not to cross that line, including by holding off on selling lethal weapons systems for use on the battlefield," according to an administration source familiar with the issue.

- Biden heads to Poland -

A top US Republican senator who also attended the Munich conference, Lindsey Graham, said it would be a serious mistake for China to provide Russia with weapons.

Doing so now, he said, would be "dumber than dirt. It would be like buying a ticket on the Titanic after you saw the movie."

Graham, known as a well-informed foreign policy hawk, also said he had strong indications that the US will soon announce plans to train Ukrainian fighter pilots, which would represent a further step in the West's gradually escalating efforts to arm Ukraine.

Graham said he believed the United States should declare Russia a state sponsor of terror for its actions in Ukraine -- which would mean that China or any other country supplying it with arms would face sanctions.

Blinken's meeting with Wang -- the highest-level encounter between the countries since US jets shot down the Chinese balloon on February 4 -- did not appear to smooth over recent friction.

"I told him quite simply that that was unacceptable and can never happen again," Blinken told CBS about the balloon incident.

Wang on Saturday dismissed the US allegations of high-altitude spying in uncharacteristically strong language, calling them "hysterical and absurd."

Blinken said that his counterpart had offered him "no apology."

The tough-sounding exchanges came a day after US Vice President Kamala Harris said in Munich that Russia had committed "crimes against humanity" in Ukraine through "widespread and systemic" attacks on the country's civilian population.

Biden will speak in Warsaw on Tuesday to hail NATO's unprecedented effort to help Ukrainians save their country as he marks the war's first year.

On the same day, President Vladimir Putin is set to give his own speech in Moscow, three days from the February 24th anniversary of Russian tanks rolling into Ukraine.