The United States on Sunday accused China of considering arming Russia in its war against Ukraine, ratcheting up tensions as the conflict hits its one-year mark this week.
Secretary of State Antony Blinken levelled the allegations as U.S.-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.
Mr. 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," Mr. 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 Mr. Wang insisted that Beijing was playing a constructive role, and would support dialogue and potential peace talks.
Appearing Sunday on ABC, Mr. Blinken emphasized that U.S. 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.