NATO on Tuesday called on Russia to prove its willingness to de-escalate the situation on the ground in Ukraine, hours after Moscow confirmed that it had withdrawn some of its troops posted along the Ukraine border.

“There are signs from Moscow that diplomacy should continue. This gives grounds for cautious optimism. But so far we have not seen any sign of de-escalation on the ground from the Russian side,” NATO Secretary General Jens Stoltenberg told reporters ahead of a two-day meeting between NATO defence ministers in Brussels, Belgium.

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“Russia has amassed a fighting force in and around Ukraine unprecedented since the cold war. Everything is now in place for a new attack. But Russia still has time to step back from the brink, stop preparing for war and start working for a peaceful solution,” Stoltenberg added.

The Ukraine situation, the “most serious security crisis we have faced in Europe for decades,” in Stoltenberg’s words, took a turn in a hopeful direction on Tuesday when the Kremlin confirmed that it had ordered some of its troops back to base from what it said were “drills” along the Ukraine border.

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Russian news agency Interfax, citing the commander of Russia’s Southern Military District, initially reported that a large number of Russian troops posted in Crimea had been ordered back to their bases in southern Russia’s Dagestan and North Ossetia areas.

The Kremlin subsequently confirmed the troop withdrawal, with spokesperson Dmitry Peskov saying, “We’ve always said the troops will return to their bases after the exercises are over. This is the case this time as well.”

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However, Ukraine remained skeptical about the credibility of Moscow’s claims, with Ukrainian foreign minister Dmytro Kuleba saying, “We won’t believe when we hear, we’ll believe when we see. When we see troops pulling out, we’ll believe in de-escalation.”

Over the past few weeks, Russia had amassed more than 100,000 troops along Ukraine’s borders, sparking fears among Western nations that an invasion is due any time. Russia, however, has maintained that it is merely seeking security guarantees and has no plans of invading its neighbour. Moscow has also accused the West of spearheading a misinformation campaign to spread hysteria.