NATO Secretary-General Jens Stoltenberg said Saturday that he had sent a letter to Russian Foreign Minister Sergei Lavrov offering more talks to defuse a possible conflict in Ukraine but warned Moscow of the dangers of making impossible security demands.

Stoltenberg said he sent the letter on Thursday urging Lavrov to agree to more talks in the format of the NATO-Russia Council, which met in January to formally discuss Moscow's calls for allies to withdraw troops from eastern Europe.

He also told the Munich Security Conference that there were no signs of a Russian pulling back from the borders of Ukraine – despite Russia's assertion this week that it had begun withdrawing troops – and that the risk of a conflict was real as Moscow's military buildup continued.

"I have invited Russia and all NATO allies to meetings in the NATO-Russia Council. And I reiterated my invitation in the letter that I sent to Minister Lavrov on Thursday," he said.

"We are extremely concerned because we see that they continue to build up, they continue to prepare. And we have never in Europe seen since the end of the Cold War, such a large concentration of combat-ready troops," he said.

In a rare admission of the limits of diplomacy, Stoltenberg also told the conference that Moscow was putting forward security demands that the Kremlin knew NATO could never meet.

In a standoff over Ukraine, Russia has sent tens of thousands of troops near the border with its neighbor while insisting it has no plans to invade. President Vladimir Putin is pressing security demands, including a block on Ukraine ever joining NATO. The bloc has said that, under United Nations treaties, every nation is free to choose its alliances.

"So that danger is now the combination of this massive military buildup, with the very threatening rhetoric, putting forward demands they know we cannot meet and say if we don't meet them, they will be military consequences," he said.

Speaking alongside Stoltenberg, European Commission President Ursula von der Leyen said Moscow's threats toward Ukraine could reshape the entire international system and cost Moscow economically.

"The world has been watching in disbelief as we face the largest buildup of troops on European soil since the darkest days of the Cold War because the events of these days could reshape the entire international order," von der Leyen said.