Russia's Foreign Ministry said on Tuesday it viewed a US decision to ease restrictions on weapons supplies to Syrian rebels as a "hostile act" which threatened the safety of Russian warplanes and military personnel, the RIA news agency reported.
RIA quoted the ministry as saying the administration of Barack Obama was attempting to complicate the situation in the world before Donald Trump takes over the White House in January.
Obama lifted some restrictions on arms deliveries to Syrian rebels earlier this month.
The Kremlin, whose air force is supporting the government of President Bashar al-Assad, said the move was risky and that weapons could end up in the hands of "terrorists".
Reporting by Peter Hobson; Editing by Andrew Osborn; Reuters
