(AFP) - Ukrainian President Volodymyr Zelensky on Sunday said he was "not ready" for talks with Russia unless its invading troops withdraw, as Kyiv investigated a deadly strike on its soldiers.

The sprawling frontline between the warring sides has remained mostly static for almost a year despite a much-touted Ukrainian counter-offensive, with Russian forces entrenched in southern and eastern Ukraine.

Officials from the United States and Europe -- Kyiv's key allies -- are reported to have suggested holding negotiations to end the grinding 20-month-old conflict, which Zelensky denied on Sunday.

The United States "know I am not ready to speak with the terrorists, because their word is nothing", he told NBC television, referring to Russia.

"They have to go out from our territory, only after that, the world can switch on diplomacy," Zelensky added.

The Ukrainian leader conceded the war was in a "difficult situation" and reiterated his need for more air defences to end Russia's control of the sky.

- Ukraine probes 'tragedy' -

A Russian missile strike killed multiple Ukrainian soldiers at what media described as an "award ceremony" this week, leading Kyiv to open a criminal investigation.

Local media said the attack, which is reported to have killed at least 20 troops, took place on Friday as a brigade gathered to receive awards in Ukraine's southern Zaporizhzhia region.

"This is a tragedy that could have been avoided," Zelensky said in an address on Sunday.

"A criminal investigation has been registered into the tragedy," he added.

"The main thing is to establish the full truth about what happened and to prevent this from happening again."

AFP was unable to immediately verify the circumstances of the strike or the number of people killed.

The Ukrainian army confirmed on Saturday that a number of soldiers from its 128th Mountain Assault Brigade had been killed in a missile strike the day before, without providing figures.

Russia said in a defence ministry briefing on Saturday that it had inflicted a "fire defeat" on a Ukrainian assault unit in Zaporizhzhia and that up to 30 people had been killed.

One Ukrainian soldier said on social media that 22 people from the brigade had been killed, criticising commanders for having held the ceremony.

The dead troops "became victims of military rudimentary traditionalism in its worst form", Ivan Savytskyy said.

- Shipyard hit -

Elsewhere on the front line, Ukrainian cruise missiles damaged a ship docked at the annexed Crimean peninsula, Russia said on Sunday, a day after Ukraine claimed the strike.

Since launching its counter-offensive earlier this year, Kyiv has ramped up attacks on the peninsula Moscow annexed in 2014, in a bid to debilitate Russia's Black Sea naval fleet.

Ukraine launched 15 cruise missiles at the B.E. Butoma shipyard based in the east coast city of Kerch on Saturday, with air defences shooting down 13, Russia's defence ministry said.

"As a result of being hit by an enemy cruise missile, a ship located at the plant was damaged," it added, without specifying the damage or naming the vessel.

Debris from the downed missiles also fell on a nearby dock, but no one was injured, Crimea's Russian-installed governor Sergei Aksyonov said.

Ukrainian and Russian attacks in and around the Black Sea have intensified since Moscow's withdrawal from the Black Sea grain deal, which aimed to ensure safe passage to civilian ships.

© Agence France-Presse