Ukrainian President Volodymyr Zelenskyy says Russia is planning to call up more troops for a major new offensive, even as Moscow was facing some of its biggest internal criticism of the war over a strike that killed scores of fresh conscripts.

Kyiv has been saying for weeks that Russian President Vladimir Putin plans to order another mass conscription drive and shut his borders to prevent men from escaping the draft.

"We have no doubt that the current masters of Russia will throw everything they have left and everyone they can round up to try to turn the tide of the war and at least delay their defeat," Zelenskyy said in his nightly video address on Tuesday.

"We have to disrupt this Russian scenario.... Any attempt at their new offensive must fail."

Russia's Defence Ministry on Wednesday blamed mobile phone use by its soldiers for a Ukrainian strike on New Year's Eve that it said had killed 89 servicemen — the deadliest incident Moscow has acknowledged for its troops since the start of the war in February 2022.

If Russia is planning a new mobilization, the deaths of scores of conscripts on New Year's Eve could undermine morale. Hundreds of thousands of men fled Russia when Putin ordered the first call-up of reservists since the Second World War in September after military setbacks.

Soldiers' widows group speaks out

Putin said last month there was no need for further mobilization.

But in a sign the Kremlin may now be considering one, a little-known group claiming to represent widows of Russian soldiers called on Tuesday for Putin to mobilize millions of men. The Kremlin has not commented on that appeal.

"We ask our President, Vladimir Vladimirovich Putin, to allow the Russian Army to carry out a large-scale mobilization," the Soldiers' Widows of Russia group said in a post on the Telegram messaging service.

• Russia blames 'massive,' illicit cellphone usage by its troops for Ukraine strike that killed 89

Russia has effectively shut down all direct opposition to the war, with open criticism banned by severe media rules. But it has given comparatively free rein to pro-war bloggers, some with hundreds of thousands of followers on social media.

Many are increasingly vocal about what they consider a half-hearted and incompetently led campaign — and they've expressed anger this week over the strike that killed Russian troops housed in a vocational school in Donetsk province on New Year's Eve.

Criticism has been directed at military commanders rather than at Putin, who has not commented publicly on the attack.