VINNYTSIA: Russian missiles struck a Ukrainian city far behind the frontlines in an attack Kyiv officials said was a war crime that killed at least 23 people while there were more signs of progress in efforts to unblock Ukrainian grain exports.

The strike on Vinnytsia on Thursday, which Ukraine said had been carried out with Kalibr cruise missiles launched from a Russian submarine in the Black Sea, followed a breakthrough in talks between Moscow and Kyiv on resuming Ukrainian grain shipments and underscored how far the two sides remain from a peace settlement. 

The United States took steps on Thursday to facilitate Russian food and fertilizer exports by reassuring banks, shipping and insurance companies that such transactions would not breach Washington's sanctions on Moscow. 

Enabling those Russian exports is a key part of attempts by United Nations and Turkish officials to broker a package deal with Moscow that would also allow for shipments of Ukraine grain from the Black Sea port of Odesa, which have been blockaded by the war.

The war in Ukraine has sent prices soaring for grains, cooking oils, fuel and fertilizer, stoking a global food crisis. Negotiators hope a deal will be signed next week.

But prospects for peace remained dim. Ukrainian President Volodymyr Zelenskiy called Russia a "terrorist" state, urged more sanctions against the Kremlin and said that the death toll in Vinnytsia could rise from 23.

"Unfortunately, this is not the final number. Debris clearance continues. Dozens of people are reported missing. There are seriously injured (people) among those hospitalized,” he said in a video address.

Russia reiterated that it does not target civilians and said its attack struck a military training facility.

"The Russian Federation only strikes military objects in Ukraine and the strike in Vinnytsia was against the house of officers where the armed forces of Ukraine were being trained," Russian diplomat Evgeny Varganov said at the United Nations on Thursday.

Reuters could not independently verify battlefield accounts.

Ukraine's state emergency service said three children, including a 4-year-old girl named Lisa, were among the fatalities and 71 people were hospitalized and 29 others missing.

It posted a photograph on its Telegram channel of a toy kitten, a toy dog and flowers lying in the grass. "The little girl Lisa, killed by the Russians today, has become a ray of sunshine," it said.

Zelenskiy told an international conference aimed at prosecuting war crimes in Ukraine that the attack had been mounted on "an ordinary, peaceful city".

Russia, which launched what it called its "special military operation" against Ukraine on Feb. 24, says its it uses high-precision weapons to degrade Ukraine's military infrastructure to protect its own security.

Vinnytsia, a city of 370,000 people about 200km (125 miles) southwest of the Ukrainian capital Kyiv, hosts the command headquarters of the Ukrainian Air Force, according to an official Ukrainian military website, a target which Russia used cruise missiles to try to hit in March, the Ukrainian air force said at the time. 

Video footage showed thick black smoke billowing out of a tall building, while photographs posted online by the State Emergency Service showed grey smoke rising later from the twisted remains of burned-out cars and smouldering rubble.

One showed an abandoned, overturned pram lying on the street.

"No other state in the world poses such a terrorist threat as Russia,” Zelenskiy said.

FIGHTING ON THE EASTERN FRONT

The United States and more 40 other countries agreed on Thursday to coordinate investigations into suspected war crimes in Ukraine.

Russia denies accusations of war crimes, and Dmitry Medvedev, a former president who is now deputy chairman of Russia's Security Council, has said that attempts by the West to punish a nuclear power such as Russia for the conflict in Ukraine risk endangering humanity. 

The Kremlin has said that Russia is ready to halt what the West calls Moscow's unprovoked war of aggression if Kyiv agrees to its conditions.

Russian Deputy Foreign Minister Andrey Rudenko said on Thursday that Moscow would respond positively should Kyiv be ready to resume peace negotiations, the Interfax news agency reported. Kyiv would have to affirm its non-aligned and non-nuclear status and formally recognise existing territorial realities, Rudenko was cited as saying.

Specifically, he said that would mean recognising that Crimea, annexed by Moscow in 2014, was under Russian control, and that two self-proclaimed Russian-backed statelets in eastern Ukraine were no longer under Kyiv's remit.

Ukraine has repeatedly said it is unwilling to concede any territory to a country it calls a hostile occupier and has said it plans to take back any land lost by force.

Daniil Bezsonov, a Russian-backed official in the self-proclaimed Donetsk People's Republic, said the statelet's armed forces and Russia were focusing their fire in eastern Ukraine on the towns of Siversk and Soledar.

The Russian plan, he said, was to seize the two towns and then move forward to attack the cities of Sloviansk and Kramatorsk from the east.

The Ukrainian military, which reported Russian shelling of Siversk, Soledar and Kramatorsk, said it was holding the line on all fronts and repelling all attempted assaults.