Al-Qaeda's affiliate in Syria has vowed to launch a new offensive "within the next 48 hours" as the first Russian military forces began to pull out of the country.

Hours after Vladimir Putin's shock announcement that he was withdrawing most Russian troops from Syria, the al-Nusra Front said it planned to step up its attacks and retake lost territory.

It is clear that Russia has suffered defeat, and within the next 48 hours Al-Nusra will launch an offensive in Syria," a commander of the group told AFP.

If the offensive does materialise, it could be the first test of whether Russia will leave its allies in the Syria to fight on their own or else keep up close air support of embattled government troops.

Meanwhile mixed signals began to emerge from Moscow as Russia's deputy defence minister said airstrikes would continue despite Mr Putin's Monday announcement.

"It is still early to talk about victory over terrorism," said Nikolai Pankov, during a ceremony at Russia's airbase in Hmeymim, Syria.

"The Russian aviation group has the task to continue carrying out strikes on terrorist facilities."

Even as some Russian aircraft began to leave, there were reports that other Russian bombing raids attacking ISIS forces in the historic city of Palmyra.

The US and Britain are treating the Russian announcement with caution. Mr Putin and President Barack Obama spoke last night about what the US termed "a partial withdrawal of Russian forces from Syria".

Staffan de Mistura, the UN envoy on Syria and the chief broker of the peace talks, called the Russian announcement "a significant development, which we hope will have a positive impact on the progress of the negotiations in Geneva".

Some Western diplomats also said there were some reasons for quiet optimism that the Russian decision and the surprisingly durable ceasefire agreement could lead to progress in Geneva, Telegraph reported.

212-11