In the face of significant and unusual deployment of Russian troops on Ukraine's borders, NATO Secretary-General Jens Stoltenberg told Russia on Monday that the western military alliance was standing by Ukraine.

Stoltenberg asked Russia to be honest about military activity in order to calm tensions and prevent an escalation, emphasizing that the most important thing right now was to keep things from spiraling out of hand.

"We must be clear-eyed and honest in our assessment of the issues we confront. And what we're seeing is a massive, large-scale military build-up in Russia," Stoltenberg said during a press conference in Brussels with Ukraine's Foreign Minister Dmytro Kuleba.

"We observe an unusual concentration of forces, and we know that Russia has been willing to deploy these types of military capabilities before to conduct aggressive measures against Ukraine," Stoltenberg said.

Russian army deployments in recent days have sparked worries of an impending military assault. Such ideas have been rebuffed by Moscow as provocative, and the NATO transatlantic alliance has increased its activity in the region.

Despite the fact that most countries continue to regard Crimea as Ukrainian, Russia annexed the peninsula in 2014 and claims ownership of the waters surrounding it.

Separatists backed by Russia seized control of Ukraine's eastern Donbass region the same year, and soldiers from both sides have died on a regular basis since then.

Stoltenberg pointed out that the troop border build-up, which Ukraine estimated at 100,000 last week, was risky since it lowered the amount of warning time if Russia decided to perform an aggressive military move against Ukraine.

When asked how he would describe the process by which Russia is deploying its military equipment towards Ukraine, a NATO source stated that large military equipment such as tanks, self-propelled artillery, and infantry fighting vehicles is being transported at night to avoid tell-tale images appearing on social media.

Lithuanian Foreign Minister Gabrielius Landsbergis told members of the media during the sidelines of a meeting with his European Union peers on Monday that the tense situation could go "either way."

Ukraine's defense ministry said around 90,000 Russian troops are stationed near their border and in rebel-controlled territories in the country's east. The Russian 41st army has remained in Yelnya, a town 260 kilometers (160 miles) north of the Ukrainian border, reports said.

Meanwhile, the West cannot rule out a Russian strike on Ukraine while the international spotlight is focused on the Belarus migration problem or Russia establishing a permanent military presence in Belarus, Landsbergis said.