Russia has kept a considerable military presence in conflict-torn Syria to bolster the regime of President Bashar al-Assad, NATO chief Jens Stoltenberg said Thursday.

"Despite the announced partial withdrawal we see that Russia maintains a considerable military presence in support of the Assad regime in Syria," he said at a press conference in Turkey.

A month ago Russian President Vladimir Putin surprised the West by ordering the "main part" of his forces to pull out.

Despite Moscow having signed on to efforts to promote a political settlement in Syria, US officials have complained that Russian warplanes appear to be flying in support of Syrian forces attacking rebel positions in Aleppo.

Since the Kremlin said it was scaling down its air presence in the country, regime forces -- backed up crucially by Russian firepower -- have scored some of their most dramatic successes in areas not covered by a February ceasefire.

The landmark partial ceasefire, which was negotiated by the US and Russia, had dramatically curtailed violence across much of Syria and raised hopes that a lasting deal could be struck to end the bloodshed.

But a new round of fierce fighting last week around Aleppo overshadowed peace talks in Geneva aimed at ending the country's five-year civil war.

Stoltenberg said the Syrian ceasefire "is under strain" but "remains the best basis for a negotiated peaceful solution to the crisis".