Russian air strikes in Syria are “undermining” efforts to find a political solution to the war, Nato’s secretary general has said.
Jens Stoltenberg said the raids were mainly targeting the Syrian opposition.
Moscow insists it is battling “terrorism”.
This week, fledgling peace talks were suspended as Syrian government forces, backed by the Russian military, launched a fresh offensive against rebels in Aleppo.
Thousands of civilians are reported to be fleeing Syria’s largest city.
- Who’s to blame for peace talks’ suspension?
- Displaced Syrians struggles to survive
- The story of the conflict
“The intense Russian air strikes mainly targeting opposition groups in Syria are undermining the efforts to find a political solution to the conflict,” Mr Stoltenberg said.
He said the air strikes posed a particular problem for Nato member Turkey, which is already embroiled in a row with Russia after Turkey shot down a Russian jet it accused of violating its airspace.
Increased Russian activity “creates risks and heightens tensions and is of course a challenge for Nato”, Mr Stoltenberg said.
On Thursday, Turkey warned up to 70,000 people might be heading to its border from Aleppo, but on Friday a BBC correspondent at the border town of Kilis saw no indication yet of large numbers of refugees.
Separately, the United Nations told Reuters news agency that at least 15,000 people had fled Aleppo.
The Syrian government is meanwhile reported to have made further gains on the ground, seizing the town of Ratyan, north of Aleppo.
Earlier in the week, it claimed a major victory by breaking the rebel siege of two towns in Aleppo province, severing an opposition supply line from Turkey to Aleppo city.
A spokesman for aid group Mercy Corps, David Evans, said the main humanitarian route to Aleppo was now cut off, warning: “It feels like a siege of Aleppo is about to begin.”
What is the Syria conflict?
Why is there a war in Syria?
Anti-government protests developed into a civil war that, four years on, has ground to a stalemate, with the Assad government, the so-called Islamic State group, an array of Syrian rebels and Kurdish fighters all holding territory.
Who is fighting whom?
Government forces concentrated in Damascus and the centre and west of Syria are fighting the jihadists of Islamic State and al-Nusra Front, as well as less numerous so-called “moderate” rebel groups, which are strongest in the north and east. These groups are also battling each other.
What’s the human cost?
More than 250,000 Syrians have been killed and a million injured. Some 11 million others have been forced from their homes, of whom four million have fled abroad – including growing numbers who are making the dangerous journey to Europe.
How has the world reacted?
Iran, Russia and Lebanon’s Hezbollah movement are propping up the Alawite-led Assad government, while Turkey, Saudi Arabia and Qatar back the more moderate Sunni-dominated opposition, along with the US, UK and France. Hezbollah and Iran are believed to have troops and officers on the ground, while a Western-led coalition and Russia are carrying out air strikes.