KYIV, Ukraine (AP) — Unauthorised use of cell phones by Russian soldiers led to a deadly Ukrainian rocket attack on the facility where they were stationed, according to the Russian military, as it raised the death toll from the weekend attack to 89.

General Lieutenant Sergei Sevryukov said in a statement late Tuesday that phone signals allowed Kyiv’s forces to “determine the coordinates of the location of military personnel” and launch a strike.

The Russian military is taking unspecified measures to “prevent similar tragic incidents in the future,” Sevryukov said, and promised to punish officials responsible for the blunder.

The attack, one of the deadliest on the Kremlin’s forces since the start of the war over 10 months ago, occurred one minute into the new year, according to Sevryukov.

It was the latest blow to the Kremlin’s military prestige as it struggles to progress with its invasion of its neighbour, and stirred renewed criticism inside Russia of the way the war is being conducted amid a successful Ukrainian counteroffensive.

Ukrainian forces fired six rockets from a US-provided HIMARS multiple launch system at a building where the soldiers were stationed. Two rockets were downed but four hit the building and detonated, prompting the collapse of the structure.

Details of the strike have trickled out in recent days.

UK intelligence officials said Wednesday that Moscow’s “unprofessional” military practices were likely partly to blame for the high casualty rate on the troops.

“Given the extent of the damage, there is a realistic possibility that ammunition was being stored near to troop accommodation, which detonated during the strike, creating secondary explosions,” the UK Defense Ministry said in a Twitter post.

In the same post, the ministry said that the building struck by Ukrainian missiles was little more than 12 kilometers (7.5 miles) from the front line, within “one of the most contested areas of the conflict,” in the Donetsk region.

“The Russian military has a record of unsafe ammunition storage from well before the current war, but this incident highlights how unprofessional practices contribute to Russia’s high casualty rate,” the update added.

Meanwhile, the Kremlin-appointed leader of the Donetsk region, one of four that Moscow illegally annexed in September, on Wednesday praised the “courage and true heroism” of the dead Russian soldiers.

Denis Pushilin said in a Telegram post that some of those killed tried to pull their comrades from the burning building.

In Samara, in southwestern Russia, locals on Tuesday gathered for an Orthodox service in memory of the dead. The service was followed by a minute’s silence, and flowers were laid at a Soviet-era war memorial, the state RIA Novosti agency reported. Unconfirmed reports in Russian-language media said the victims were mobilized reservists from the region.

The Russian Defense Ministry, in a rare admission of losses, initially said the strike killed 63 troops. But as emergency crews sifted through the rubble of the building, the death toll mounted. The regiment’s deputy commander was among the dead.

Unconfirmed reports put the death toll much higher.