Ukraine Presses U.N. Over ‘Nuclear Blackmail’ at Russian-Occupied Plant

“Particular attention was paid to the topic of Russia’s nuclear blackmail at the Zaporizhzhia NPP,” Mr. Zelensky wrote on social media. He said the two men also discussed allegations that Ukrainian citizens were being forcibly deported to Russia and the treatment of captured Ukrainian soldiers.

Mr. Guterres called for the plant to be demilitarized. Russia has said pulling its forces out of the facility would threaten the nuclear plant’s security.

After meeting with Turkish President

Recep Tayyip Erdogan,

Mr. Zelensky said they had discussed ways to protect Ukrainian grain that is being exported, as well as other security issues. Ankara helped broker with the U.N. a deal to lift a Russian naval blockade on Ukrainian exports, which had led to food shortages throughout the Middle East and Africa.

“This is a strong message of support from such a powerful country as Turkey,” Mr. Zelensky wrote on Telegram.

The Turkish president has sought to position himself as a mediator in the war, with Turkey hosting two rounds of unsuccessful peace talks between Ukraine and Russia. Mr. Erdogan has said he hopes the U.N.-backed initiative that led to the resumption of Ukraine’s Black Sea grain exports earlier this month could be a starting point for a broader peace between Russia and Ukraine.

At a news conference following the talks, he said he had “reiterated our support for the sovereignty and territorial integrity of Ukraine.” He added: “I have been preserving my belief that the war would come to an end at the negotiation table.”

Ukraine has exported 622,000 tons of grain and other food products from the three ports covered by the export agreement, the Turkish Defense Ministry said Thursday.

During the news conference, Mr. Guterres said, “there is no solution to the global food crisis without insuring full global access to Ukraine’s food products and Russian food and fertilizer.” Global wheat prices, he said, have fallen up to 8% since the accord was signed.

Turkish military officers are helping to monitor implementation of the agreement alongside their Ukrainian and Russian counterparts and U.N. officials stationed at a control center that was set up in Istanbul in July. Four more ships loaded with agricultural products sailed from Ukrainian ports on Wednesday under the deal, according to Turkish officials.

Mr. Erdogan is increasingly posing as a friend to both sides in the Ukraine conflict. Turkey has delivered weapons to Ukraine, including armed drones that have been instrumental in Ukraine’s battle against the Russian invasion. In February, Turkey also invoked its rights under an international treaty to bar additional Russian warships from the Black Sea.

His visit to Ukraine comes less than two weeks after a visit to Russia where he held talks on the Ukraine war and the grain initiative with Russia’s President

Vladimir Putin.

“This will be another opportunity for Mr. Erdogan to be active in this mediation process,” said

Aydin Sezer,

a former diplomat who served in Turkey’s embassy in Moscow. “Erdogan is now the only person who is credited by the Kremlin when it comes to Ukrainian business.”

Turkish and Ukrainian officials also signed a memorandum of understanding calling for Turkey to participate in Ukraine’s postwar reconstruction. The first project being considered under the agreement is the reconstruction of a bridge connecting Kyiv with the towns of Irpin and Bucha, where Russian soldiers carried out mass killings in March, the Ukrainian presidency said.

“Turkey is our strategic ally. We are grateful to our Turkish partners for their willingness to cooperate in the recovery of the infrastructure destroyed by Russia,” said Ukraine’s Infrastructure Minister

Oleksandr Kubrakov

according to the Ukrainian president’s office.

Earlier on Thursday, the Ukrainian military’s Southern Command said that it had struck an ammunition depot in the village of Bilohirka, near the front line of fighting in the Kherson region. The rocket strike is the latest in a series of attacks that have targeted logistics in the Russian-occupied south—part of a strategy to starve Russian troops in the region of supplies and force them to withdraw from the territory they are holding west of the Dnipro River.

On Thursday evening, an ammunition depot in Russia’s southern Belgorod region bordering Ukraine ignited in flames, the region’s governor wrote on his social media pages. Videos shared on social media showed dark clouds of smoke rising into the sky, accompanied by explosions. The governor,

Vyacheslav Gladkov,

said there were no casualties and residents of two nearby villages were being evacuated. There was no information about the cause of the fire. Mr. Gladkov said authorities were investigating.

Around the same time, videos on social media showed billowing clouds of smoke and fire in the city of Kadiivka in the eastern Luhansk region, which Russian forces captured in early July. Russian-installed authorities in Luhansk said that Ukrainian forces had struck the city with U.S. Himars mobile rocket launchers.

A day earlier, the Ukrainian military posted video to social media that appeared to show the aftermath of a long-range rocket strike on Nova Kakhovka, in the Kherson region. And on Tuesday, pro-Ukrainian saboteurs destroyed an ammunition depot in Crimea, which Russia seized in 2014. Video on social media Thursday also showed large explosions overnight in Russian-occupied Amvrosiivka, in the eastern Donetsk region; Ukrainian officials didn’t immediately comment on the cause.

Late Thursday, air-defense systems were activated in the city of Kerch, Oleg Kryuchkov, the adviser to the Russian-appointed head of Crimea, wrote on his Telegram channel. A day earlier,

Mykhailo Podolyak,

an adviser to Mr. Zelensky, had suggested that the Kerch bridge connecting Crimea to mainland Russia is a legitimate military target for Ukraine. Mr. Kryuchkov wrote Thursday that Kerch and the bridge weren’t in danger.

As Ukrainian strikes inside Russian-held territory increase, Russian forces are attempting to crack down on pro-Ukrainian insurgents. A Ukrainian army veteran was arrested in the Kherson region on suspicion of sending locations of Russian troops and bases to Ukrainian forces, Russian state-run news agencies reported on Thursday. In addition, Russia’s FSB intelligence agency on Wednesday said it had detained six Russian citizens in Crimea who belonged to a cell that spread what it called terrorist ideology with the support of Ukrainian emissaries, according to Russian state news agency RIA Novosti.

Russia has said it would give International Atomic Energy Agency inspectors access to the Zaporizhzhia nuclear plant—but only if they come via Russian-controlled territory and not through Kyiv, a plan that Ukraine opposes.

The Russian Defense Ministry on Thursday said Ukraine was planning a false flag provocation for Friday at the plant to frame the occupying forces. Maj. Gen.

Igor Konashenkov,

a Russian Defense Ministry spokesman, didn’t provide evidence to support the claim. The Russian-installed head of the occupied territories of Zaporizhzhia, meanwhile, said a plan was in place to evacuate residents in case of an attack on the plant. Kyiv didn’t immediately respond to the claim.

Ukraine’s energy transmission network*

Russia’s Defense Ministry also said Thursday that Moscow would consider shutting down the plant if the situation surrounding the facility continues to deteriorate.

The Ukrainian government, international nuclear-power watchdogs and the plant’s staff have accused Russia of stealing Zaporizhzhia’s power by severing its connection to Ukraine’s remaining territory.

In Kharkiv, in northeastern Ukraine, a Russian missile hit a residential building in the Saltivka neighborhood on Wednesday night, killing seven people and injuring at least 17 more, according to the city’s mayor. More missiles launched from Russia hit the city early Thursday morning, killing two more people. Russia’s Defense Ministry said its forces were targeting foreign fighters.

Russia’s Defense Ministry said Thursday it has deployed three MiG-31 combat jets armed with hypersonic Kinzhal ballistic missiles to the Russian exclave of Kaliningrad, a chunk of Russia wedged between North Atlantic Treaty Organization members Lithuania and Poland, according to Russian state news agencies. Such missiles, when fired from jets, have farther reach than the ground-launched missiles already deployed in Kaliningrad.

—Nikita Nikolaienko contributed to this article.

Write to Ian Lovett at ian.lovett@wsj.com, Jared Malsin at jared.malsin@wsj.com and Evan Gershkovich at evan.gershkovich@wsj.com