US President Joe Biden's administration rejected calls for a "no-fly" zone over Ukraine, arguing that this would lead to escalation.

Declaring a "no-fly" zone would require enforcement, and that would "essentially mean the US military would be shooting down planes, Russian planes," White House spokeswoman Jen Psaki said on MSNBC Monday morning.

"That is definitely escalating and would potentially put us in a place in a military conflict with Russia. That's something the president doesn't want to do," she added, noting the administration has repeatedly said that US troops will not be fighting Russian forces barring a NATO Article 5 situation.

The Biden administration wants to soften the rhetoric and escalate after Russian President Vladimir Putin ordered the country's deterrence forces to be placed on high alert.

Psaki said that the US had its own preparations and its own capabilities and capacities to defend the United States, but the country had not raised its alert level.