In this Aug 23, 2020 photo, Russian PM Mikhail Mishustin and Russian Defence Minister Sergei Shoigu, right, attend the opening ceremony of the 6th International Military Technical Forum 'Army 2020' and Army Games in the military Patriot Park outside Moscow. (PHOTO / AFP)

MOSCOW/WASHINGTON – Russia's defense minister on Tuesday accused US bombers of rehearsing a nuclear strike on Russia from two different directions earlier this month and complained that the planes had come within 20 km of the Russian border. 

But the Pentagon said its drills were announced publicly at the time and adhered to international protocols. 

Moscow's accusation comes at a time of high tension with Washington over Ukraine, with US officials voicing concerns about a possible Russian attack on its southern neighbor – a suggestion the Kremlin has dismissed as false. 

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Moscow has in turn accused the United States, NATO and Ukraine of provocative and irresponsible behavior, pointing to US arms supplies to Ukraine, Ukraine's use of Turkish strike drones against separatists in eastern Ukraine, and NATO military exercises close to its borders. 

Sergei Shoigu reportedly said Russian air defense units had spotted and tracked the US strategic bombers and taken unspecified measures to avoid any incidents 

Russian Defense Minister Sergei Shoigu said that Moscow had noted a significant increase in the activity by US strategic bombers, which he said had carried out 30 flights close to Russia this month. That, he said, was 2.5 times more than the same period last year. 

Shoigu complained in particular of what he said was a simulated US nuclear strike against Russia earlier this month. 

"The defense minister underlined that during the US military exercises 'Global Thunder', 10 American strategic bombers rehearsed launching nuclear weapons against Russia from the western and eastern directions," Shoigu was quoted as saying in a defense ministry statement. 

"The minimum proximity to our state border was 20 km." 

Shoigu was quoted as saying that Russian air defense units had spotted and tracked the US strategic bombers and taken unspecified measures to avoid any incidents. 

The Pentagon pushed back. 

"These missions were announced publicly at the time, and closely planned with (Strategic Command), (European Command), allies and partners to ensure maximum training and integration opportunities as well as compliance with all national and international requirements and protocols," said Lieutenant Colonel Anton Semelroth, a Pentagon spokesperson. 

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The top Russian and US military officers, Chief of General Staff Valery Gerasimov and Chairman of the Joint Chiefs of Staff Mark Milley, spoke by telephone on Tuesday but neither side disclosed the contents of the conversation. 

Global Thunder, which this year put US nuclear-capable B-52 bombers through their paces, is the US Strategic Command's annual nuclear and command exercise designed to test and demonstrate the readiness of US nuclear capabilities. 

President Vladimir Putin referenced the apparent episode briefly last week, complaining of Western strategic bombers carrying "very serious weapons" close to Russia. He said the West was taking Moscow's warnings not to cross its "red lines" too lightly.