Russia on Thursday launched large-scale snap military drills, putting its troops on full combat readiness in districts bordering Ukraine and the Baltic states, the defence minister said.

"In accordance with the decision of the Supreme Commander-in-Chief of Armed Forces of the Russian Federation (President Vladimir Putin), a spot check started today," Defence Minister Sergei Shoigu said in televised comments.

Troops in the country's southern, central and western military districts, as well as the air force, northern fleet and paratroopers were in "full combat" mode as of 0400 GMT, he said.

The drills -- which are taking place in military districts that encompass the contested Crimea peninsula, the region that borders Ukraine and NATO members Estonia, Latvia and Lithuania, as well as territory spanning from the Ural mountains to central Siberia -- are set to last until the end of the month.

Shoigu did not reveal how many troops are taking part in the exercises.

Relations between NATO and Russia have soured since Moscow annexed the Crimea peninsula of Ukraine in March 2014, and eastern European countries are worried that they too could be targets of Russian aggression.

NATO vowed at a summit in Warsaw in July to bolster its eastern flank to counter a resurgent Russia, agreeing to deploy four battalions in Poland and the Baltic states.

Moscow slammed the decision, accusing NATO of working to counter a "non-existent threat."

Since Moscow annexed Crimea from Ukraine in March 2014, Russia has stepped up its military presence in the peninsula, which is home to its Black Sea fleet.

Russia's most advanced air defence system, the S-400, was delivered to Crimea earlier this month.