Russia launched air strikes in Syria in support of long-term ally President Bashar al-Assad in September 2015.

Russian support helped Assad's forces reclaim swathes of territory once in the hands of opposition fighters and Islamic extremists.

Russian Defence Minister Sergei Shoigu told an annual meeting of top brass that Russia's air force had seen a drastic reduction in the number of sorties in Syria.

"Flights have been reduced from 100-110 per day to two to four sorties per week, mainly for additional reconnaissance," Shoigu said, without giving a specific timeframe for the reduction.

Shoigu told the meeting, also attended by President Vladimir Putin, that Russia "had completed the withdrawal of its main forces from Syria," not including those based at its Hmeimim airbase and Tartus naval base.

Shoigu announced a year ago that Russia had formally ceased its military operation in Syria after helping to clear it of jihadists.

However operations have continued, with Russia saying in November its planes had carried out air strikes on Syrian "terrorist groups" in the buffer zone in the northwestern province of Idlib.

Syria's war erupted in 2011 following a regime crackdown on anti-government protests, with more than 360,000 killed and much of the country left in ruins.

While much of Syria was once rocked by heavy fighting, separate offensives by pro-government and US-backed Kurdish forces have dislodged opposition and jihadist fighters from most of their former strongholds, leaving the northwestern province of Idlib as the last major rebel bastion.