Turning on their cell phones and making massive use of them allowed the enemy to locate its target, officials said.

Ukraine says 400 soldiers were killed and another 300 wounded in the attack on a college for conscripts in Makiivka, in the occupied Donetsk area.

It is the largest number of deaths Russia has acknowledged in the war.

Russia said that at 00:01 Moscow time on New Year's Day, six rockets were fired from a US-made Himars rocket system at a vocational college, two of which were shot down. Moments earlier, President Vladimir Putin had given his annual New Year's address on Russian TV. 

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The deputy commander of the regiment, Lt.Col. Bachurin, was among those killed, the ministry of defence said in a statement on Wednesday. A commission was investigating the circumstances of the incident.

But it was "already obvious" that the main cause of the attack was the use of cell phones by troops in range of Ukrainian weapons, despite this being banned, it added.

"This factor allowed the enemy to locate and determine the coordinates of the location of military personnel for a missile strike."

Lt.Gen. Sergei Sevryukov said officials found responsible by the investigation would be brought to justice and "all the necessary measures are currently being adopted to prevent this kind of tragic incident in the future".

The vocational college was packed with soldiers at the time - men believed to have been among the 300 000 called up in Putin's partial mobilisation in September. Ammunition was also being stored close to the site, which was reduced to rubble.

The head of Russia's proxy authority in the Donetsk region, Denis Pushilin, praised the heroism of those caught up in the missile strike, who he said had tried to pull comrades out of the building. Some of those returning to the building had died in the process, he added.

BBC