Ukraine's president urged civilians on Saturday to evacuate the frontline Donetsk region, the scene of fierce clashes with the Russian military, as Kyiv called on the Red Cross and UN to gain access to its soldiers being held by Moscow's forces, reported AFP.

The eastern Donetsk region has faced the brunt of Russia's offensive since its assault on Kyiv failed weeks into the military operation launched on February 24.

Zelensky warned in his daily address that thousands of people, including children, were still in the region's battleground areas, with six civilians killed and 15 wounded on Friday, according to the Donetsk governor.

"There's already a governmental decision about obligatory evacuation from Donetsk," Zelensky said, underscoring authorities' calls to leave the besieged region in recent weeks.

"Leave, we will help," Zelensky said. "At this stage of the war, terror is the main weapon of Russia."

Zelensky, in his address, again pressed the international community, especially the United States, to have Russia officially declared a "state sponsor of terrorism".

He reiterated the call a day after a jail holding Ukrainian prisoners of war in Kremlin-controlled Olenivka was bombed, leaving scores dead, with Kyiv and Moscow trading blame.

On Saturday, Ukrainian human rights official Dmytro Lubinets said on national television he had asked the International Committee of the Red Cross (ICRC) and the United Nations Human Rights Monitoring Mission to go to Olenivka.

The ICRC has made a request but has not yet obtained authorisation from the Russians, he said.

- 'Egregious provocation' -

Russia's defense ministry accused Kyiv of striking the Olenivka prison with US-supplied long-range missiles, in an "egregious provocation" designed to stop captured soldiers from surrendering.

It said Saturday that the dead included Ukrainian forces who had surrendered after weeks of fighting off Russia's brutal bombardment of the sprawling Azovstal steelworks in the port city of Mariupol.

The defense ministry said 50 Ukrainian prisoners were killed and 73 were taken to hospital with serious injuries.

"All political, legal and moral responsibility for this bloody massacre of Ukrainians lies with Zelensky personally, his criminal regime and Washington, which backs them," it said.

Zelensky laid the blame squarely on Russia.

"This was a deliberate Russian war crime, a deliberate mass murder of Ukrainian prisoners of war," he said.

Members of the Azov regiment were among those who surrendered at Azovstal.

Azov regiment commander Mykyta Nadtochiy said he considered the attack on the jail to have been "an act of public execution".

- Gazprom cuts off Latvia -

Also on Saturday, Russian energy giant Gazprom suspended gas supplies to Latvia, in the latest tightening of gas provision to European Union states, which have accused Russia of squeezing supplies in retaliation for Western sanctions imposed over Moscow's operation on Ukraine.

Conexus Baltic Grid confirmed to Latvia's LETA news agency that Gazprom had informed it of the suspension of deliveries, but said other suppliers were continuing them.

"Today, Gazprom suspended its gas supplies to Latvia... due to violations of the conditions" of purchase, the company said on Telegram.

Latvia's Economy Minister Ilze Indriksone told LETA that his country "was not counting on natural gas flows from Russia."

Gazprom drastically cut gas deliveries to Europe via the Nord Stream pipeline on Wednesday to about 20 percent of its capacity. It had reduced gas flows to Europe twice in June.

The Russian state-run company had earlier announced it would choke supply to 33 million cubic meters a day -- half the amount it has been delivering since service resumed last week after 10 days of maintenance work.

Gazprom cited the halted operation of one of the last two working turbines for the pipeline due to the "technical condition of the engine".

Kremlin spokesman Dmitry Peskov has blamed EU sanctions for the limited supply.

The EU this week agreed a plan to reduce gas consumption in solidarity with Germany, where the Nord Stream pipeline runs to, warning of Russian "blackmail".

- Grain fields set alight -

Russian strikes continued to rain down on Ukrainian towns and cities on Saturday.

Ukrainian authorities said Russian bombardments targeting the south and east of the country had left one dead in southern Mykolaiv and one dead in eastern Bakhmut.

The death toll from a strike on a Mykolaiv bus stop on Friday climbed to seven after two men died in hospital, he added.

In the eastern city of Kharkiv, three Russian S-300 missiles struck a school, mayor Igor Terekhov said on Telegram, adding that the main building was destroyed.

A Ukrainian spokesman said his country's forces had set fire to grain fields around Mariupol so they could not be used by Russian forces.

"The Mariupol resistance forces set fire to the fields with grain so that it would not be stolen by the occupiers," Sergiy Bratchuk, a spokesman for the Odessa regional military administration said.