The head of NATO warned Tuesday that, in addition to deploying forces and military equipment into eastern Ukraine, Moscow is now engaged in a “serious military buildup” on the Russian side of the border with Ukraine.

“We call on Russia to pull back its troops,” NATO Secretary General Jens Stoltenberg said as he arrived for a meeting with EU defense ministers in Brussels.

Mr. Stoltenberg, a former prime minister of Norway who took up the NATO leadership post last month, said the alliance has intelligence showing a flow of Russian troops, tanks, artillery and advanced air defense systems into eastern Ukraine.

Reuters reported that he said the Russian actions violate a Sept. 5 cease-fire agreement — the so-called “Minsk protocol” — that German leaders negotiated between Russia and Ukraine.

Ukrainian forces for months have clashed with Moscow-backed separatist rebels around the city of Donetsk in eastern Ukraine and the recent troop buildups, along with an uptick in violence in the area, suggest the tenuous cease-fire has all but collapsed.

Last week, NATO’s supreme allied commander, U.S. Air Force Gen. Philip Breedlove, cited intelligence showing “columns of Russian equipment, primarily Russian tanks, Russian artillery, Russian air defense systems and Russian combat troops, entering into Ukraine.”

Gen. Breedlove said Russia also appeared to be reinforcing its military bases in the Crimean Peninsula with equipment that is “capable of being nuclear,” but added that NATO wasn’t certain nuclear weapons were being deployed.

Russia this year annexed the peninsula, which had been a region of Ukraine.

The latest developments have prompted fears in Washington that Moscow may now be preparing to try and repeat its Crimea action by annexing a wider swath of eastern Ukraine.

But Russian officials have denied charges they are expanding Moscow’s military presence in the area.

Maj. Gen. Igor Konashenkov, a spokesman for the Russian Defense Ministry, called Gen. Breedlove’s claims unfounded last week.

“We’ve stressed many a time there are no real facts behind the acts of shaking the air by Brussels officials,” said Gen. Konashenkov, who accused the NATO commander of making “alarmist anti-Russian allegations.”

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