Russian President Vladimir Putin ordered troops placed on full combat readiness in snap drills in western Russia, as Defense Minister Sergei Shoigu warned the country was facing new threats to its security.

Some 38,000 troops, 41 warships, 15 submarines and 110 aircraft are involved in the exercises, Shoigu said on Monday, according to a Russian Defense Ministry statement. “New challenges and threats to military security demand a further increase in the military capabilities of the armed forces,” Shoigu said, the Interfax news service reported.

Russia is “deeply concerned about the increase in exercises of NATO countries close to our borders,” Russian Deputy Foreign Minister Aleksey Meshkov said on Monday, according to Tass news service. It’s “particularly surprising that this takes place in the northeastern region of Europe, which is the most stable in security terms, not only on our continent but possibly in the world,” he said.

The exercises began a day after Putin disclosed in a Russian television documentary that he was ready to put nuclear forces on alert when he annexed Ukraine’s Crimea peninsula a year ago. Crimea held a disputed referendum on joining Russia on March 16, 2014, two days before Putin authorized the annexation, triggering the worst crisis in relations between Russia and the U.S., Europe and the North Atlantic Treaty Organization since the Cold War.

The almost yearlong conflict that has killed more than 6,000 people in eastern Ukraine erupted weeks later between pro- Russian separatists and government troops. A cease-fire negotiated by the leaders of Ukraine, Russia, Germany and France in Minsk, Belarus, on Feb. 12 is gradually taking hold.

Exercises taking place over six days to March 21 will check the military’s ability to strengthen forces on the islands of Novaya Zemlya and Franz Josef Land, defend Russia’s borders by land and air, including in the country’s far north, and destroy the naval grouping of a simulating enemy, Shoigu said, according to Interfax.