KYIV, Ukraine — Unauthorized use of cell phones by Russian soldiers led to a deadly Ukrainian rocket attack on the facility where they were stationed, the Russian military said late Tuesday, raising the death toll from the weekend attack to 89.

Gen. Lt. Sergei Sevryukov said in a statement that phone signals allowed Kyiv’s forces to “determine the coordinates of the location of military personnel” and launch a strike. Sevryukov said unspecified measures were being taken to “prevent similar tragic incidents in the future” and promised to punish officials responsible for the infraction.

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.

Ukrainian forces fired six rockets from a U.S.-provided HIMARS multiple launch system at a building “in the area of Makiivka” where the soldiers were stationed. Two rockets were downed but four hit the building and detonated, prompting the collapse of the structure. The Russian Defense Ministry initially said the strike killed 63 troops. But as emergency crews sifted through the rubble of the building, the death toll has grown to 89, Sevryukov said on Tuesday. The regiment’s deputy commander was among the dead.

Other, unconfirmed reports put the death toll much higher.

The Strategic Communications Directorate of Ukraine’s armed forces claimed Sunday that around 400 mobilized Russian soldiers were killed in a vocational school building in Makiivka and about 300 more were wounded. That claim couldn’t be independently verified. The Russian statement said the strike occurred “in the area of Makiivka” and didn’t mention the vocational school.

The attack marked yet another setback for the Kremlin’s bogged-down war effort in Ukraine, undermined by a successful Ukrainian counteroffensive. It stirred renewed criticism inside Russia of the way the war is being conducted.

Unconfirmed reports in Russian-language media said the victims were mobilized reservists from the region.