WASHINGTON (Nexstar) — The situation between Russia and Ukraine is intensifying. American officials are monitoring events closely as the world fears an imminent attack by Russia on Ukraine.

The British government said Russia is plotting to overthrow the government in Ukraine and install pro-Kremlin leaders, something US Secretary of State Antony Blinken said, “We’ve been warning about just this type of tactic for weeks.”

More than 120,000 Russian troops are stationed along Ukraine’s border.

On CNN’s “State of the Union,” Blinken said the United States will not tolerate any Russian forces entering Ukraine.

“That would trigger a swift, severe and united response from the US and from Europe,” Blinken said.

American officials said a Russian invasion of Ukraine would be the most serious threat to global peaces since World War II.

“If he’s able to go into Ukraine and there’s very little pushback from the United States or from NATO, it allows him to move into other countries in Eastern Europe,” Iowa Sen. Joni Ernst said.

While urging diplomacy, the US said NATO will not meet Russia’s demands to ban Ukraine from membership. Russia’s other demands include that NATO pull back forces from Eastern Europe.

Russia denies the Kremlin is planning to replace Ukraine’s leader.

Some members of Congress, like Rep. Michael McCaul, said they US should not wait to find out.

“The noose is tightening around Ukraine,” McCaul said. “President Zilensky, as you mentioned, said he want the sanctions now.”

But Blinken said the US would lose leverage if sanctions are implemented too soon.