"Russia and Iran were the main backers of the Assad regime, and they share the responsibility for the crimes committed against the Syrian people," Rutte said.

"They also proved to be unreliable partners, abandoning Assad when he ceased to be of use to them."

The head of the Western military alliance said the end of Assad's rule "is a moment of joy but also uncertainty for the people of Syria and the region."

"We hope for a peaceful transition of power and an inclusive Syrian-led political process," Rutte said.

"We will be closely watching to see how the rebel leaders conduct themselves during this transition. They must uphold the rule of law, protect civilians and respect religious minorities."

The end of Assad's rule followed a 13-year-old civil war sparked by a brutal crackdown on pro-democracy protests.

The war killed more than 500,000 people and forced half the pre-war population to flee their homes, many millions of them abroad.

NATO is facing off against Russia and Iran over the the Kremlin's war in Ukraine. The alliance is focused on Europe and North America and has only a very limited role in the Middle East.