NATO chief Jens Stoltenberg Sunday defended the alliance's decision to refrain from stepping into the war in Syria, saying doing so would only make matters worse.

All 28 NATO members belong to the US-led coalition battling the Islamic State group but they are not directly involved in the Syrian conflict.

"We are experiencing in Syria a horrible human catastrophe. Sometimes it is right to deploy militarily -- such as in Afghanistan," Stoltenberg told Bild am Sonntag.

"But sometimes the costs of a military operation is higher than its benefit. Looking at Syria, NATO partners came to the conclusion that a military deployment would only make a terrible situation worse," he said.

"We would risk turning it into a bigger regional conflict. Or more innocent people could die. A military deployment is not always the solution," he warned.

The West has come under fire from some quarters over its failure to halt the carnage in Syria.

Trapped civilians and rebels in besieged Aleppo were on Sunday waiting desperately for evacuations to resume, as the UN Security Council was due to vote on sending observers to the flashpoint city.

France is pushing for the monitors, arguing that an international presence would prevent Aleppo from turning into another Srebrenica, where thousands of Bosnian men and boys were massacred in 1995 when the town fell to Bosnian Serb forces during the Balkan wars.