NATO head Jens Stoltenberg said Friday that Russia's air strikes in Syria targeting rebel forces are "undermining" efforts to find a non-military solution to the war.
"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 ministers.
Syrian peace talks in Geneva earlier this week broke up acrimoniously as long-time Moscow ally President Bashar al-Assad launched a fresh offensive against rebel forces in Aleppo with massive Russian backing.
The UN Security Council is due to meet later Friday to discuss the situation, with the negotiations on hold until February 25.
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.
Stoltenberg said NATO strongly supported efforts to end the war in Syria, which has cost more than 250,000 lives and displaced half the population, many of them fleeing to Europe in its worst migrant crisis since World War II.
He said the air strikes and Moscow's military build-up in Syria and the eastern Mediterranean were a challenge for the US-led alliance and especially for key member Turkey, whose airspace Russian planes have violated.
"The increased Russian presence (and) air activity in Syria is also causing increased tensions and violations of Turkish airspace," he said.
"This creates risks and heightens tensions and is of course a challenge for NATO," he added.
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 since.