A renewed buildup of Russian troops near the Ukrainian border has raised concern among some officials in the United States and Europe who are tracking what they consider irregular movements of equipment and personnel on Russia’s western flank.

The officials, who spoke on the condition of anonymity because they weren’t authorized to discuss the matter publicly, said the troop movements have reignited concerns that arose in April, when the largest buildup of troops by Russia near the Ukrainian border in years sparked an international outcry.

The renewed movements of Russian forces in the area come as the Kremlin embraces a harder line on Ukraine. Russian officials from President Vladimir Putin on down have escalated their rhetoric in recent months, attacking Kyiv’s Western ties and even questioning its sovereignty. Putin has warned that any expansion of NATO military infrastructure on Ukrainian territory represents a “red line” for Moscow.

The situation also comes as the simmering 7½-year conflict between Ukrainian forces and Russian-backed separatists in the eastern Donbas region enters a new stage. On Oct. 26, Ukraine’s military confirmed it had used a Turkish-made drone against a position in Donbas, the first time Kyiv has employed the technology in combat, prompting an outcry from Moscow.

Relations between Moscow and NATO are especially tense. Russia suspended its mission to NATO in Brussels on Oct. 18 after the alliance expelled eight members of the Russian mission on accusations of espionage.

What the Russian forces gathering near the Ukrainian border are doing is unclear.

Videos have surfaced on social media in recent days showing Russian military trains and convoys moving large quantities of military hardware, including tanks and missiles, in southern and western Russia.

“The point is: It is not a drill. It doesn’t appear to be a training exercise. Something is happening. What is it?” said Michael Kofman, director of the Russia studies program at the Virginia-based nonprofit analysis group CNA.

Officials in the United States and Europe began noticing the movements particularly in recent weeks, after Russia concluded a massive joint military exercise with Belarus known as Zapad 2021 on its western flank in mid-September.

According to Kofman, publicly available satellite imagery shows that forces from Russia’s 41st Combined Arms Army, normally based in the Siberian city of Novosibirsk, didn’t return to Siberia after the exercises, and instead linked up with other Russian forces near the Ukrainian border. Kofman also said footage posted online appears to show that Russia’s 1st Guards Tank Army, an elite unit based outside Moscow, is moving personnel and materiel toward Ukraine.

Oleksiy Danilov, secretary of Ukraine’s national security and defense council, said in a statement that after the conclusion of the Zapad 2021 exercises, Russia left military equipment, as well as control and communications centers, at training sites along the Ukrainian border.

Danilov estimated that the number of Russian troops deployed around the Ukrainian border at 80,000 to 90,000, not including the tens of thousands stationed in Crimea.

Moscow has prompted alarm by amassing large contingents of troops near the Ukrainian border in the past, only to subsequently withdraw those forces and quell fears of a renewed invasion of Ukraine.

Ukrainian officials see such moves as a way for Moscow to keep the situation tense and uncertain. Since 2015, when the front lines of the conflict more or less froze in place, the buildups haven’t led to a mass territory-gaining offensive by Russia or the separatist forces it backs.

Kofman said this time the situation may merit particular attention, however, because Russia appears to have lowered its threshold for what would prompt the Kremlin to act in Ukraine.

“I think you have to step back and look at this past year holistically, and if you do, you will know that the Russian tone and messaging on Ukraine has changed dramatically,” he said.

In statements over the course of the year, Putin and other top Russian officials have said the expansion of NATO activities in Ukraine represents a “red line” for Moscow, whereas previously they cited NATO membership for Ukraine as a move they couldn’t abide, Kofman said.

“It appears that Russia is shifting the goal post of what is acceptable,” he said.

Putin outlined that view in an article in July, claiming Ukraine was being functionally controlled by Western nations to foment anti-Russian sentiment.

“We will never allow our historical territories and people close to us living there to be used against Russia,” Putin wrote. “And to those who will undertake such an attempt, I would like to say that this way they will destroy their own country.”

In comments to the Valdai Discussion Club this month, Putin said formal NATO membership for Ukraine may never happen, “but military expansion on the territory is already underway, and this really poses a threat to the Russian Federation.”

Dmitry Medvedev, former Russian president and now the deputy head of Russia’s security council, called Ukraine a “vassal state” that is “under direct foreign control” in an Oct. 11 article in the Kommersant newspaper.

Previously, Belarusian President Alexander Lukashenko, an ally of Putin, accused the United States of building NATO military bases in Ukraine using training centers as a cover.

U.S. troops have been training Ukrainian forces in western Ukraine for years, an initiative undertaken by Washington and its NATO allies after Russia annexed Crimea in 2014. Ukrainian forces also have participated in joint exercises with the United States and its NATO allies. The United States has provided Ukraine with Javelin antitank weaponry but has not taken any active role in fighting.

Kofman said Russia this year suddenly invested a large amount of money in developing a ready reserve for its military and also ordered forces to practice anti-drone and anti-Javelin maneuvers relevant to the conflict in Ukraine, attracting the attention of military analysts.

Putin has condemned comments by U.S. Secretary of Defense Lloyd Austin, who during a visit to Ukraine this month described Russia as an obstacle to peace and said NATO membership remained open to Kyiv. Austin, referring to Russia, said no nation had a right to veto membership.

Russian Ministry of Foreign Affairs spokeswoman Maria Zakharova on Thursday accused Ukraine of having plans to retake control of the Donbas region by force. Kremlin spokesman Dmitry Peskov a day earlier said sales of Turkish drones to Ukraine potentially “destabilized” the situation.

German and French Foreign Ministry officials expressed concern about Ukraine’s use of Turkish-made drones, calling for de-escalation. But Ukrainian officials said the country was exercising its right to self-defense after a Russian howitzer attack killed a Ukrainian serviceman and wounded another.

“When the Ukrainian army feels the need to defend its land, it does so. And it will further act under this principle,” Ukrainian President Volodymyr Zelensky said in comments to journalists Friday.

“We are not mounting an offensive, we are just responding,” Zelensky added.

Ukrainian Foreign Minister Dmytro Kuleba said Russia is “actively spreading fakes about Ukraine allegedly preparing an offensive or other nonsense. For the record, Ukraine does not prepare any offensive in the Donbas.”

Kuleba noted that Moscow has left in place hardware and personnel that were sent near the Ukrainian border during the spring buildup and the September exercises.

The Russians “pulled back only a tiny part of the armada,” Kuleba said.

The U.S. Embassy in Kyiv warned Friday that Russia had repeatedly deployed howitzer artillery and drones against Ukrainian forces “in direct violation” of a July 2020 cease-fire agreement.

“Official Russian rhetoric suggesting Ukraine is aggravating the situation is not only misleading, it serves to escalate tensions,” the U.S. statement said.

John Hudson contributed to this report.