KYIV, Ukraine—Ukraine called for Russia to be designated a state sponsor of terrorism as it continued to push for an investigation into the killing of scores of Ukrainian prisoners of war in a rocket strike on Russian-held territory, while Moscow accused Ukraine of targeting the facility with Western-made weapons.

On the energy front, Russian state-owned energy producer Gazprom PJSC on Saturday halted gas deliveries to Latvia, saying without elaborating that Riga had violated contractual terms. The announcement comes days after Gazprom said gas exports through the vital Nord Stream pipeline linking Russia to Germany would drop to about a fifth of the pipe’s capacity, blaming sanctions-related problems with turbines that have already reduced flows.

Gazprom has already cut off supplies to other countries like Poland, Finland and the Netherlands because they refused to pay in rubles instead of euros. It was not clear what the effect of the move would be on Latvia, which had been reducing imports of Russian gas and will bar all further imports on Jan. 1, 2023, Latvian natural gas company AS Latvijas Gāze said before the Gazprom announcement.

Kyiv’s calls for Russia to be branded a state sponsor of terrorism follow the U.S. Senate’s approval two days earlier of a nonbinding resolution calling on Secretary of State Antony Blinken to approve such a measure. Some U.S. officials say it could severely limit prospects for diplomatic engagement with Russia, but Ukraine says recent evidence of Russian atrocities—including footage it says shows torture of Ukrainian POWs—only increases the urgency of such a move.

“Every occupier who humiliates Ukrainians, who tortures and murders, should know that there will be a price for this,” Ukrainian President Volodymyr Zelensky said late Friday. “Even if some of the Russian murderers expect that they won’t be made to answer, that they can hide somewhere, know this: They will answer come what may. Geography, time, borders and walls will not stand in the way of a just retribution.”

Ukrainian soldiers returned Friday from the front line on the outskirts of Bakhmut, eastern Ukraine.

Photo: bulent kilic/Agence France-Presse/Getty Images

Russia has said the prisoners were killed in a Ukrainian strike using Western-provided Himars rockets, but Ukraine’s General Staff said that the country’s armed forces hadn’t launched any rocket or artillery strikes in the Olenivka area where the prisoners were held that night, and that they use high-precision Western weapons only against Russian military targets.

Mr. Zelensky urged intervention from the International Committee of the Red Cross, which had brokered the evacuation in May of some 2,000 Ukrainian fighters defending the Azovstal steel plant in Mariupol, many of whom were moved to Russian-controlled facilities like Olenivka as Moscow’s forces consolidated control of the city. The Ukrainian president said that as a guarantor of their humane treatment in accordance with the Geneva Conventions, the ICRC should speak out to condemn Russia’s alleged actions.

Kyiv has asked the ICRC to help secure access for its own investigators, including human-rights ombudsman Dmytro Lubinets. Late on Friday, the ICRC issued a statement that didn’t blame either Ukraine or Russia, saying it had requested access to the site of the attack so that it can ensure the wounded receive lifesaving treatment.

“All prisoners of war, wherever they are held, are protected under international humanitarian law. They are no longer part of the fight and should not be attacked,” it said.

Oleksiy Reznikov, Ukraine’s defense minister, described the killing of Ukrainian POWs as “Ukraine’s Katyn,” a reference to the mass executions of some 22,000 Poles by Joseph Stalin’s Soviet Union in 1940.

Residents of Slovyansk have endured fighting over the city, a target in Russia’s bid to capture eastern Ukraine’s Donetsk region.

Photo: bulent kilic/Agence France-Presse/Getty Images

Ukraine’s General Staff accused Russia of staging the attack on the facility in Olenivka, a town in the eastern Donetsk region controlled by Russian forces, and killing at least 50 Ukrainian prisoners it had taken captive “to hide the torture of prisoners and executions committed there.”

Russian state news agency RIA Novosti published footage showing what it said were Himars rocket fragments at the site of the attack, without showing the numbers apparently written on them or panning out to show where exactly they had been located. The agency also posted separate footage it said showed the destroyed facility and its heavily damaged roof with charred bodies strewn on the floor of the building.

The Institute for the Study of War, a Washington, D.C.-based think tank that publishes daily analyses of the war in Ukraine, said the damage shown in the footage from Olenivka is “not the sort of damage that a Himars strike would likely have caused.”

Vyacheslav Volodin, the speaker of Russia’s lower house of parliament, said on Saturday that Mr. Zelensky and the U.S. organized the attack in Olenivka to prevent the prisoners from being prosecuted in Russia and revealing incriminating details about the Ukrainian leadership. “This was done with one aim: to prevent a new Nuremberg trial,” he said. The Kremlin has smeared the Azovstal fighters as neo-Nazis as part of a propaganda campaign to discredit Kyiv.

Kyiv has said the idea that it would deliberately target its own prisoners of war held captive by Russia defies logic, and later on Friday, Mr. Zelensky described the strike as a “deliberate Russian war crime, a deliberate mass killing of Ukrainian POWs.”

The remains of a college building heavily damaged by a Russian missile strike in Kharkiv.

Photo: STRINGER/REUTERS

Just hours after news emerged of the attack, Russia’s embassy in the U.K. wrote on

Twitter that “Azov militants deserve execution…They deserve a humiliating death.” The tweet prompted numerous calls for its account to be suspended and for Twitter to remove the post, which was later updated with a note saying it violates the platform’s rules on hateful conduct.

Ukrainian military intelligence has accused Russian mercenary company Wagner Group of striking the facility under the personal direction of Yevgeny Prigozhin, a businessman close to the Kremlin who Western officials say finances Wagner, and said the strike wasn’t coordinated with the Russian Defense Ministry. Mr. Prigozhin’s representatives didn’t respond to a request for comment.

Russia meanwhile continued strikes on Ukrainian cities. Officials in Dnipropetrovsk region said 80 Grad missiles had been fired at the city of Nikopol, damaging a power substation and leaving 1,000 residents without electricity. In Mykolaiv, a front-line city in south Ukraine that comes under regular attack, Gov. Vitaly Kim said one person died and six were injured in strikes on a residential area.

In the southern region of Kherson, Ukraine has continued to mount counteroffensives as it prepares to ultimately try to retake the regional capital, also named Kherson, the first major Ukrainian city to be captured by Russia since it launched its invasion on Feb. 24.

The head of the U.K.’s foreign-intelligence service, Richard Moore, tweeted on Saturday that the Russians were “running out of steam” in Ukraine.

—Mauro Orru contributed to this article.

Write to Matthew Luxmoore at Matthew.Luxmoore@wsj.com