A second group of Russian warplanes flew out of Syria on Wednesday in line with Moscow's decision to withdraw most of its forces, as US Secretary of State John Kerry prepared a rare trip to Moscow to discuss the conflict.

SU-25 combat aircraft and IL-76 transport planes pulled out of Russia's Hmeimim base in Syria, the defense ministry said in a statement.

The departure follows that of a first group of aircraft which arrived back in Russia on Tuesday to a hero's welcome.