NATO head Jens Stoltenberg said Friday that Russia's air strikes in Syria targeting rebel forces are "undermining" efforts to find a peaceful solution to the five-year war.
Syrian peace talks in Geneva earlier this week broke up acrimoniously as Moscow's long-time ally President Bashar al-Assad launched a fresh offensive against rebel forces in Aleppo with massive Russian backing.
"What we have seen is that the intense Russian air strikes mainly targeting opposition groups in Syria are undermining the efforts to find a political solution to the conflict," Stoltenberg said as he arrived for talks in Amsterdam with EU defence and foreign ministers.
The NATO chief also criticised Russia's military build-up in Syria and in the eastern Mediterranean as stoking regional tensions, especially with key alliance member Turkey.
"This creates risks and heightens tensions and is of course a challenge for NATO," he said.
Turkey shot down a Russian fighter jet along its Syrian border in late November and the two sides have been engaged in a bitter war of words ever since.
With the Syrian peace talks put on ice until February 25, the UN Security Council is due to take up the crisis later Friday in hopes of getting the latest peace initiative back on track.
On Thursday, US Secretary of State John Kerry said he had warned Moscow to stop targeting the Syrian opposition in what he described as a "robust" phone call with Russian Foreign Minister Sergei Lavrov.
French Foreign Minister Laurent Fabius was equally critical of Russia's role in Syria, charging Moscow with "torpedoing" the peace talks.
"There cannot be political discussions if one of the parties is engaged in assassinating the other," Fabius said on his arrival at the talks in Amsterdam.
The conflict in Syria has cost more than 250,000 lives and displaced half the population, many of them fleeing to Europe in its biggest migrant crisis since World War II.
The deteriorating situation in Syria and its regional impact have added to EU and NATO concerns over Libya, which has been in turmoil since the alliance backed the 2011 overthrow of long-time dictator Moamer Kadhafi.
Warring factions have divided up the oil-rich country, with the Islamic State group now building a presence there as EU efforts to forge a government of national unity appear stalled.
Stoltenberg said NATO was "following very closely" the situation in Libya, stressing the importance of agreeing a new government "because that will be an important first step to be able to fight IS."
The United States warned Thursday that there could now be 5,000 IS fighters in Libya, about double previous estimates, even as their numbers fall in Syria and Iraq.