Russia's air force has started military drills in central and western regions of the country, a move which could spark further fears that Moscow is ready to flex its military muscle in neighbouring Ukraine, the Interfax news agency has said.
The drills will continue until Friday, air force chief Igor Klimov was reported as saying, and will involve more than 100 fighter jets and helicopters.
Pro-Russian separatists in eastern Ukraine have been battling the Kiev government since April, a conflict that has claimed at least 1,129 civilian casualties, according to a UN estimate.
Ukraine and Western countries have accused Russia of providing the rebels with equipment and expertise, a claim that the Russian government has repeatedly denied.
Russia's Ministry of Defence could not be reached for comment.