Following a recent spike in airborne confrontations between the U.S. and Russian militaries, three B-52 Stratofortress bombers joined Ukrainian fighter jets for training Friday.

The bombers, from the 5th Bomb Wing, Minot Air Force Base, North Dakota, “conducted vital integration training with Ukrainian fighters inside Ukraine's airspace,” European Command said in a press release. Officials said the bomber mission, part of a long-planned deployment of six B-52s to RAF Fairford, United Kingdom, “provided partners valuable mid-air training.”

U.S. Air Force B-52H Stratofortress bombers flew from the United Kingdom to Ukraine airspace. Three B-52Hs, with the call signs Julia 51, 52, and 53 departed RAF Fairford in the United Kingdom on Sept. 4, 2020. After arriving there, they orbited at the edge of the Russian-occupied Crimean Peninsula and near areas under the control of Kremlin-supported separatists. There are conflicting reports about whether the third bomber took part in the mission to Ukraine, but only two of them were ever visible on online flight tracking software Regardless, the bombers subsequently returned to Fairford, where a total of six of them – all of which are capable of carrying nuclear weapons – have been forward-deployed as part of a Bomber Task Force mission since Aug. 22.

“The mission demonstrated how forward-located aircraft and crews, such as those in the B-52 units, enable collective defence capabilities and provide the U.S., NATO Allies and partners strategic and operational breadth to deter Russia and assure Allies and partners,” the release said.

Earlier Friday, aircraft spotters on social media noted two B-52s headed southeast toward Crimea — land claimed by Ukraine that was annexed by Russia in 2014.

The flights come one week after a B-52 formation flew across all 30 NATO countries in a single-day mission — dubbed “Allied Sky' — meant to showcase solidarity with partners in the region.

But during the bombers' flight over the Black Sea Aug. 28, two Russian Su-27 Flanker jets turned on their afterburners, coming within 100 feet of one of the B-52s.