Russia has upped its saber rattling in Eastern Europe and the Arctic, a move that has put the Biden administration on alert.

In the past two weeks, Moscow has moved to test Washington and its allies on land, in the air and at sea with a buildup of military equipment in eastern Ukraine, military flights near Alaskan airspace and submarine activity in the Arctic.

“I think we’ve been very clear about the threats that we see from Russia across domains. … We’re taking them very, very seriously,” Pentagon press secretary John Kirby said Wednesday.

The Defense Department is overly observant of Russian activity after fighting resumed between Moscow-backed separatists and Ukrainian soldiers in eastern Ukraine, ending a cease-fire the two groups made last summer. Twenty Ukrainian soldiers have been killed in the skirmishes since the start of 2021. The two sides have been fighting since 2014 when Moscow seized and annexed Crimea from Ukraine, a conflict that Kyiv asserts has killed 14,000 people since its start.

Russian jets and bombers have also frequently flown near allied airspace, forcing NATO jets to scramble to respond 10 times on Monday alone. In addition, in late March, three Russian nuclear ballistic missile submarines simultaneously broke through several feet of ice in the Arctic in a military drill, a maneuver that comes as the Kremlin has moved to raise its defenses in the Arctic.

Russia’s aggressive actions have prompted U.S. European Command to raise its alert status to its highest level, and the activity in Ukraine, in particular, has prompted Biden’s top national security leaders to call their Ukrainian counterparts and other leaders in the region.

Joint Chiefs of Staff Chairman Gen. Mark Milley on Wednesday spoke by phone with Ukraine Chief of General Staff Lt. Gen. Ruslan Khomchak as well as Russia’s top officer, Chief of the General Staff Gen. Valery Gerasimov.

A day later, Defense Secretary Lloyd Austin called his Ukrainian counterpart, Minister of Defense Andriy Taran, to “discuss the regional security situation” and condemn “recent escalations of Russian aggressive and provocative actions in eastern Ukraine.”

National security adviser Jake Sullivan last week also spoke with his Ukrainian counterpart, as did Secretary of State Antony Blinken, who said he discussed “ways of strengthening security cooperation” with Ukrainian Foreign Minister Dmytro Kuleba. All leaders have pledged that Washington would stand by Kyiv.

But Russia on Friday warned NATO against deploying troops to Ukraine, threatening that such actions would escalate tensions and that Moscow would be forced to respond.

“There is no doubt such a scenario would lead to a further increase in tensions close to Russia’s borders. Of course, this would call for additional measures from the Russian side to ensure its security,” Kremlin spokesman Dmitry Peskov told reporters on Friday.

The head of U.S. Northern Command, Air Force Gen. Glen VanHerck, said the posturing is due to the United States and Russia being back in “great power competition,” much like during the Cold War.

“Clearly, Russia is trying to reassert on a global stage their influence and their capabilities,” VanHerck told reporters on Wednesday.

“The difference between the past and now is the intercepts are more complex – multiaccess, multiplatform – and oftentimes they’ll enter the [air defense identification zone] and stay for hours,” he added.

In 2020, the North American Aerospace Defense Command, which is in charge of defending North American airspace, responded to more Russian military flights off the coast of Alaska than in any year since the end of the Cold War.

Moscow’s bellicose stance appears to have continued into 2021, giving President Biden a foreign policy challenge in the early days of his administration.

Last month, when asked if he thought Russian President Vladimir Putin was a “killer” Biden answered, “I do,” adding that the Russian leader will “pay a price” for the country’s influence operation targeting the 2020 election and other cyberattacks.

Backing up his hardline stance, Biden in late February approved another $125 million worth of security aid to Ukraine to defend its borders against Russia. The money adds to the more than $2 billion in lethal assistance the U.S. government has sent to the country since Crimea’s annexation.

It remains to be seen whether fighting in eastern Ukraine intensifies or how NATO would respond, but the Pentagon has made clear it is on alert.