Vladimir Putin’s threatened retaliation against Ukraine over its drone attack on Russia’s bomber fleet has not happened yet in earnest, despite heavy bombardment of Kharkiv and Kyiv the past two days, and is likely to be a significant, multi-pronged strike soon, US officials have told Reuters. One senior western diplomat anticipated a “huge, vicious and unrelenting” assault by Moscow.

Michael Kofman, a Russia expert at the Carnegie Endowment for International Peace, said Russia might seek to punish Ukraine’s SBU domestic security agency which orchestrated last weekend’s assault, possibly employing intermediate-range ballistic missiles, as well as targeting defence manufacturing sites. Still, Kofman suggested Russia’s options for retaliation may be limited, as it was already throwing a lot of its military might at Ukraine. “In general, Russia’s ability to substantially escalate strikes from what they are already doing – and attempting to do over the past month – is quite constrained.”

The Ukrainian drone attack – called Operation Spiderweb – likely damaged about 10% of Russia’s strategic bomber fleet and hit some of the aircraft as they were being prepared for strikes on Ukraine, according to a German military assessment. Maj Gen Christian Freuding told a YouTube podcast that “more than a dozen aircraft were damaged, TU-95 and TU-22 strategic bombers as well as A-50 surveillance planes”. He said “only a handful” of the A-50s exist and can now no longer be used for spare parts.

Despite the losses, Freuding did not see any immediate reduction of Russian strikes against Ukraine, noting that Moscow retained 90% of its strategic bombers, which can launch ballistic and cruise missiles in addition to dropping bombs. “But there is, of course, an indirect effect as the remaining planes will need to fly more sorties, meaning they will be worn out faster, and, most importantly, there is a huge psychological impact.”

Freuding said Russia had felt safe in its vast territory, which also explained why there was little protection for the aircraft. “After this successful operation, this no longer holds true. Russia will need to ramp up the security measures.”

Russia and Ukraine have accused each other of thwarting a large-scale prisoner exchange that was agreed in the last week, with Kyiv denying Russian allegations that it had indefinitely the postponed prisoner swaps while accusing Moscow of “playing dirty games”. At talks in Istanbul on Monday, Kyiv and Moscow agreed to release more than 1,000 people on each side, while Russia said it would also hand back the remains of 6,000 killed Ukrainian soldiers.

Russia’s defence ministry said in a statement: “The Russian side has provided the Ukrainian side with a list of 640 names, but the Ukrainian side is currently refraining from setting a date for the return of these individuals and the transfer of the corresponding number of Russian prisoners of war.”

Ukraine’s Coordination Headquarters for the Treatment of Prisoners of War said no date had been agreed for the return of bodies and that a list of names Russia said would be released did not match the terms of the agreement. Andriy Kovalenko, an official with Ukraine’s national security and defence council, said “statements by the Russian side do not correspond to reality”.