Stoltenberg, who is on a visit to Croatia's capital Zagreb, called on the Syrian regime and Russia to respect international law.

"NATO's deployment in the Aegean Sea remains important. But the international community needs to address the root causes of the crisis. We need to see a political solution to the conflict in Syria. So the Assad regime and Russia must end their offensive. Respect international law. And support UN efforts for a peaceful solution," said Stoltenberg.

Speaking about the influx of asylum seekers from Turkey to Greece, he said Greece is faced with a tough situation.

"Greece is also severely impacted by the current situation. Greece carries a heavy burden. And the challenges of migration are shared challenges that require shared solutions," said Stoltenberg.

Meanwhile, Croatian Prime Minister Andrej Plenkovic said that irregular migration had to be prevented.

"We will try to prevent the new migrant influx in cooperation with the EU and NATO," said Plenkovic.

More than 135,000 asylum seekers have crossed to Europe after Turkey opened its borders, accusing the EU of not fulfilling the promises it made as part of a 2016 agreement reached on the refugee crisis.

Turkey, which already hosts some 3.7 million Syrian refugees, more than any country in the world, says it cannot absorb another wave of migration.

An escalation in Syria's Idlib has sent a fresh wave of migrants toward Turkey's border.