MOSCOW — Russia is steadily massing its largest military presence in years near the Ukrainian border — on land and at sea — as the Kremlin tests Western support for Kyiv and its battles against pro-Moscow separatists less than three months into the Biden administration.

Russia’s motivations for the buildup are still unclear and do not necessarily signal a looming offensive, Ukrainian and Western officials said.

But moving forces from as far away as Siberia — more than 2,000 miles distance — to near Ukraine and onto the Crimean Peninsula has injected new levels of alarm in a region that has been a flash point between the West and Moscow since 2014.

In March that year, Russia annexed Crimea from Ukraine, prompting international condemnation and sanctions. The following month, war broke out in the eastern Ukrainian region of Donbas between Russian-allied separatists and Ukraine’s military.

More than 13,000 people have been killed in the fighting since then, according to the United Nations. The last bout of large-scale combat was more than four years ago, but there have been periodic exchanges of artillery fire along a front line that has barely budged.

Russia’s sudden military surge appears to be more about sending messages than launching a fresh offensive, analysts said.

For Russian President Vladimir Putin, Ukraine’s warm relations with the United States and Europe are a challenge to Moscow’s influence in the region, especially as Biden has vowed to take a harder line with the Kremlin. Ukraine’s aspirations of joining NATO are also seen by Russia as a potential threat on its doorstep.

Some of the Russian military moves have a long-term feel. Russian troops and military hardware are at a recently constructed camp near Voronezh, about 155 miles from the Ukrainian border, according to the Moscow-based Conflict Intelligence Team, which monitors Russia’s military and security services.

Russia also is relocating the 56th Guards Air Assault Brigade to Feodosia in Crimea.

“Russia is testing everyone’s nerves and declaring its position: It should remain an important player for other countries, both the United States and Ukraine,” said Ruslan Leviev, an analyst with the Conflict Intelligence Team.

“They are trying to show that Russia will not tolerate any sanctions or other actions put in place to pressure them to return Crimea to Ukraine or to change the course of things in Donbas,” he added.

Russia's move on display

The Russian military began the shift about a month ago, Leviev said. At first, the redeployment was thought to be part of planned exercises. But when the maneuvers ended in late March, the military array stayed.

Tanks have crossed the bridge connecting Russia and Crimea. Trains carrying military hardware from northern Russian regions have made multiple trips, Leviev said.

Russia’s Ministry of Defense has also said it is moving more than 10 naval vessels, including landing boats and artillery warships, from the Caspian Sea to the Black Sea, which lines Ukraine’s coast, for “exercises.” And it has all happened in the open.

The Conflict Intelligence Team has flagged about 150 videos, mainly from TikTok, showing the Russian military on the move. “It feels like the Russian Ministry of Defense wants these convoys and trains to be filmed,” Leviev said.

“Because then the message about muscle-flexing and the costs of playing with its power will reach Ukraine and Western countries through the media,” he added.

Kremlin spokesman Dmitry Peskov said Russia “is moving troops within its own territory at its own discretion, and this shouldn’t concern anyone.”

Serhii Deineko, the head of the Ukrainian State Border Guard Service, estimated that at least 85,000 Russian troops are positioned between six and 25 miles from the Ukrainian border and in Crimea.

White House press secretary Jen Psaki said Thursday that Russia now has more troops on Ukraine’s eastern border than at any time since 2014.

“We’ve asked Russia for an explanation of these provocations,” State Department spokesman Ned Price told reporters, “but more importantly, what we have signaled with our Ukrainian partners is a message of reassurance.”

One senior U.S. official said Russia’s troop movements could further set back efforts to work with Moscow on areas of mutual interest, including talks to reverse a Trump-era withdrawal from the Open Skies Treaty, an international pact that U.S. allies have argued allows for valuable transparency and dialogue between Moscow and Washington. The official spoke on the condition of anonymity to discuss sensitive U.S.-Russia issues.

Ukraine bolsters its forces

In an April 2 phone call with Ukrainian President Volodymyr Zelensky, Biden pledged “unwavering support for Ukraine’s sovereignty and territorial integrity in the face of Russia’s ongoing aggression,” according to the White House readout. On Thursday, Zelensky visited Ukrainian troops near the front lines.

The United States has sent Ukraine $4.5 billion in assistance since 2014, including two shipments of Javelin antitank missiles provided by the Trump administration. The advanced weaponry is kept far from the front lines because of U.S. concerns about provoking Russian escalation and the possibility of the weaponry falling into the hands of U.S. adversaries, according to congressional aides familiar with the situation.

The location of the missiles is kept secret from the public. Spokespeople with the State Department and Pentagon declined to say whether Ukraine had moved the missiles in response to Russia’s troop movements.

Two U.S. warships are en route to the Black Sea and will remain there until May 4, the Turkish Foreign Ministry said Friday.

Fighting in Donbas dropped off significantly after a cease-fire was introduced last summer. But clashes have flared in recent months. Ukrainian officials say that more than 20 of their soldiers have died this year — four of them in one 24-hour period.

Ukraine has called up reservists to fortify its eastern and northern borders in response to Russia’s troop movements.

“What is quite clear for me is that Zelensky and the people around him are seriously concerned about a possible invasion,” said Alyona Getmanchuk, the director of the New Europe Center think tank in Kyiv.

Zelensky appealed to NATO for membership in a call with Secretary General Jens Stoltenberg on Tuesday. Zelensky said it was the “only way to end the war” with the separatists.

But the warnings from Moscow are stark. Dmitry Kozak, the Kremlin’s chief negotiator in relations with Kyiv and the separatists, said Thursday that if Ukraine joined NATO, that would lead to the country’s “disintegration.”

“I support the opinions that also exist inside Ukraine that the start of military action would mean the beginning of Ukraine’s end,” he said.

Sharper rhetoric

In late January, Kremlin propagandist Margarita Simonyan, the editor in chief of the government-funded TV channel RT (formerly Russia Today), took the stage at the Russia Donbas Forum in Donetsk, an eastern Ukrainian border region controlled by separatists.

She called on Moscow to take a more aggressive stance with its neighbor.

“The people of Donbas want to have the opportunity to be Russian. And we must give them that opportunity,” she told the crowd. “The people of Donbas want to live at home and be a part of our great, generous motherland. And we must give them that opportunity.”

Asked the next day whether Simonyan’s words signaled plans for a Russian offensive, the Kremlin spokesman Peskov said Simonyan “in no way could be the spokesman for the official position of the Russian Federation. This issue is not on the agenda.”

Tensions between Ukraine and Russia have spiked in recent months as Zelensky moved to implement pro-Western reforms and weaken the political grip of Ukraine’s oligarchs. He sanctioned the tycoon Viktor Medvedchuk, a friend of Putin’s. Moscow also was angry that Zelensky closed several pro-Russian media outlets in February.

In a commentary for the Carnegie Moscow Center, Maxim Samorukov said “the Kremlin was well prepared to up the military ante” as Zelensky plays “the anti-Russian card.”

On Thursday, in a call with German Chancellor Angela Merkel, Putin “noted provocative actions by Kyiv, which is deliberately inflaming the situation along the line of contact,” the Kremlin said in its readout of the call, using a term to denote the front lines in eastern Ukraine. Merkel “demanded that this buildup be unwound in order to de-escalate the situation,” Berlin said in its readout of the conversation.

“Ukrainians view this buildup as a bit puzzling,” said an adviser close to Zelensky, speaking on the condition of anonymity because of the sensitivity of the issue. “The idea that Ukraine is about to launch an invasion [into Donbas] is viewed as ludicrous and a pretext for something else.”

The escalation also is a test for European governments, particularly Germany, which has been criticized by Washington for its plans to complete a pipeline to import natural gas from Russia.

“Russia is literally putting a gun to Ukraine’s head and blackmailing its neighbor with military escalation,” Omid Nouripour and Manuel Sarrazin, foreign policy spokesmen for Germany’s Greens political party, said in a statement.

That sentiment was echoed by one European diplomat, who spoke on the condition of anonymity in line with protocol to brief the news media.

“Russia’s buildup of forces is at best destabilizing and aggressive posturing, at worst the buildup to a military offensive,” he said. “The lack of a credible explanation and disinformation about Ukrainian provocation is intended to undermine Ukraine and its sovereignty.”

“Russia should not underestimate how closely this is being followed in European capitals at present,” he added.

Stern reported from Kyiv, Morris from Berlin and Hudson from Washington. Mary Ilyushina in Moscow contributed to this report.