MOSCOW • Russia's Defence Minister has accused American bombers of rehearsing a nuclear strike on the country from two different directions earlier this month, and complained that the planes had come within 20km 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 neighbour - a suggestion that the Kremlin has dismissed as false.

Russia has in turn accused the United States, Nato and Ukraine of provocative and irresponsible behaviour, pointing to US arms supplies to Ukraine, Ukraine's use of Turkish strike drones against Russian-backed separatists in eastern Ukraine, and Nato military exercises close to its borders.

Russian Defence Minister Sergei Shoigu said on Tuesday that Moscow had noted a significant increase in activity by US strategic bombers, which he said had carried out 30 flights close to Russia this month.

That, he said, was 21/2 times more than the same period last year. Mr Shoigu complained in particular of what he said was a simulated US nuclear strike against Russia earlier this month.

"The defence 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," according to a Defence Ministry statement.

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

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

The Pentagon has pushed back, with a spokesman, Lieutenant-Colonel Anton Semelroth, saying: "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."

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 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 American nuclear capabilities.

Russia's 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.

Mr Shoigu made the comments in a video conference with Chinese Defence Minister Wei Fenghe.

He said that US bomber flights close to Russia's eastern borders were also a threat to China.

"Against this backdrop, Russo-Chinese coordination is becoming a stabilising factor in world affairs," said Mr Shoigu.

Russia and China agreed at their meeting to step up cooperation between their armed forces when it came to strategic military exercises and joint patrols, the Defence Ministry said.

REUTERS