Russia’s ministry of defence threatened to increase the scale of missile strikes against the Ukrainian capital Kyiv, a day after the sinking of the Moskva, the flagship of its Black Sea fleet.

Igor Konashenkov, the ministry’s spokesman, said on Friday that Russia would intensify its attacks on targets in Kyiv in response to any further attempts by Ukrainian forces to carry out “sabotage” on Russian soil.

The threat came a day after Russia admitted it had lost the Moskva missile cruiser — arguably the biggest setback of its sputtering 50-day campaign — as it prepares for a renewed offensive in southeastern Ukraine.

Russia said on Thursday that the ship sank in a storm after a fire onboard set off its ammunition stores. Ukraine said it caused the ship to sink after hitting the Moskva with one of its cruise missiles. US defence officials said they were unable to confirm what had happened.

Overnight on Thursday, air alert sirens sounded in cities around Ukraine, and loud booms resounded in Kyiv.

The Moskva’s sinking was met with jubilation in Ukraine. Resistance to the ship became an early symbol of the fight against Russia when Ukrainian troops, ordered by the Moskva to surrender, responded by saying: “Russian military vessel, go fuck yourself.”

Oleksiy Reznikov, minister of defence, posted on Twitter a picture of himself scuba diving and said: “A ‘flagship’ Russian warship is a worthy diving site. We have one more diving spot in the Black Sea now. Will definitely visit the wreck after our victory in the war.”

Russia has not admitted that Ukraine struck the ship. Instead, it framed the strikes as a response to what it claims is a series of increasingly daring raids behind enemy lines, including an apparent attack on a railway bridge on a crucial resupply route near Belgorod.

On Friday, Russia claimed it had destroyed one of several helicopters it said were attacking a village in Bryansk, another border region, where investigators claimed two residential buildings were destroyed and eight people injured.

Ukraine has not taken responsibility for the attacks, which some analysts say president Vladimir Putin could use as a pretext for broadening the invasion.

Ukraine’s military said Russia was gathering its forces for an assault on the east of the country. “The enemy continues to focus its efforts on preparations for an offensive operation on eastern Ukraine and taking over the territory of the Donetsk and Luhansk regions and creation of a so-called ground corridor with . . . Crimea,” Col Oleksandr Motuzyanyk, a defence ministry spokesman, said in a press briefing on Friday.

He added that the Russian military continued to use rockets to hit Ukrainian civilian infrastructure, and that on Thursday two strategic bombers from airspace over Krasnodar in southern Russia launched cruise missiles into Ukrainian territory.

Motuzyanyk also said that for the first time since the beginning of the war, Russia used long-range Tupolev 2 and Tupolev bombers in the besieged city of Mariupol.

As Russia regroups for an attempt to seize the eastern Donbas border region, the Kremlin’s justifications and goals for the war continue to shift, opening the door to a renewed widening of the offensive.

Dmitry Peskov, Putin’s spokesman, told reporters on Friday that “the most important thing is to free [Ukraine] from these nationalist battalions,” marginal far-right groups that Russia falsely claims have seized control of Ukraine’s government.

“The operation is continuing and the tasks at hand are well known. They must and will be completed. There should not be any doubts,” Peskov said, according to Interfax.

Natalia Humeniuk, a spokeswoman for the Ukrainian military in the country’s south, said on Friday that Mykolaiv, a strategic southern city under Ukrainian control, was being shelled in what she claimed was retaliation for the Moskva’s sinking.

“The situation in the south of Ukraine is indeed tense,” Humeniuk said in a press briefing. “Stable but tense.” 

“When the Moscow cruiser was hit, it affected not only the ships, it affected Russia’s imperial ambitions, and we of course understand that they will not forget, they will not forgive,” Humeniuk said.