(CNSNews.com) - Linda Thomas-Greenfield, the U.S. Ambassador to the United Nations, told CNN's "State of the Union" on Sunday that Chinese support for Russia's war of aggression in Ukraine would be crossing a "red line," and she used the phrase twice:

First, she said it was "unacceptable" for China to send a spy balloon to hover over the United States.

"And it did have an impact on our relationship. But we know we have to continue communications, diplomatic communications, with the Chinese, so that we can ensure that they understand what our red lines are and not make the mistake of crossing them."

Moments later, Thomas-Greenfield was asked about the Chinese announcement that they want peace in Ukraine:

"Well, we welcome the Chinese announcement that they want peace, because that's what we always want to pursue in situations like this,” she said.

"But we also have to be clear that, if there are any thoughts and efforts by the Chinese and others to provide lethal support to the Russians in their brutal attack against Ukraine, that that is unacceptable. Again, that would be a red line."

Thomas-Greenfield would not discuss the consequences of China crossing the "red line."

"We're not going to advance and announce what we're planning to do. But we made clear to the Chinese that there will be consequences should they make that unfortunate decision."

Host Pamela Brown turned the discussion to a potential Chinese invasion of Taiwan "by 2030."

"Look, this is why we are continuing to have diplomatic conversations with the Chinese," Thomas-Greenfield said.

"We're continuing our efforts to communicate with the Chinese, despite what they did with the balloons recently, so that we have clear lines of communications with them to avoid such a thing happening. But if such -- such an event happens, we're very clearly prepared for that."

The Biden administration has labelled China a "strategic competitor."

"We are prepared to compete with the Chinese," Thomas-Greenfield said. "And we are (prepared), when necessary, to confront the Chinese. And that's what we're doing. And that's what we will continue to do to ensure that our national interests are always at the forefront."