KYIV, Ukraine — The owner of Russia's Wagner Group military contractor claimed Wednesday that his troops have extended their gains in the Ukrainian stronghold of Bakhmut, but it remained unclear how long the grinding fight might go on.

Meanwhile, U.N. Secretary-General Antonio Guterres visited Kyiv for talks with President Volodymyr Zelenskyy on extending an agreement that allows Ukraine to ship grain from its Black Sea ports and permits Russia to export food and fertilizers.

The battle for the city the Ukrainians have dubbed “fortress Bakhmut” has become emblematic of the way each side has tried to wear down the other. Russian forces must go through Bakhmut to push deeper into parts of the Donetsk province they do not yet control, though Western officials say that capture of the city is unlikely to change the course of the war.

The battle for Bakhmut has lasted six months and reduced the city with a prewar population of more than 70,000 to a smoldering wasteland. It’s not clear which side has paid a higher price.

Wagner owner Yevgeny Prigozhin, whose troops have spearheaded the fight in Bakhmut, said they have taken full control of all districts east of the Bakhmutka River that crosses the city. The city's center lies west of the river.

Neither Russian nor Ukrainian officials commented on Prigozhin's claim. The Institute for the Study of War, a Washington-based think tank that closely monitors the fighting, said Russian forces were likely in control in the areas cited by Prigozhin following a Ukrainian withdrawal.

Russian troops have enveloped the city from three sides, leaving only a narrow corridor leading west. The only highway west has been targeted by Russian artillery fire, forcing Ukrainian defenders to rely increasingly on country roads, which are hard to use before the muddy ground dries.

Zelenskyy vowed Monday not to retreat from Bakhmut after chairing a meeting with his top generals.

Russian Defense Minister Sergei Shoigu said Tuesday that seizing the city would allow Russia to press its offensive farther into the Donetsk region, one of the four Ukrainian regions that Moscow illegally annexed in September.

In a blustery video statement recorded near a World War II monument in Bakhmut, Prigozhin echoed that rationale, saying the prospective Russian push would make “the entire world shudder.”

NATO Secretary-General Jens Stoltenberg acknowledged that the Russians could seize the city soon.

“What we see is that Russia is throwing in more troops, more forces, and what Russia lacks in quality they try to make up in quantity," he told reporters on the sidelines of an EU defense ministers meeting in Stockholm. "They have suffered big losses, but at the same time we cannot rule out that Bakhmut may eventually fall in the coming days.”

But like other Western officials, he played down the significance of Bakhmut's potential capture, arguing that this “does not necessarily reflect any turning point of the war, and it just highlights that we should not underestimate Russia.”

The Ukrainian military has already strengthened defensive lines west of Bakhmut to block the Russian advance, including in the nearby town of Chasiv Yar that sits on a hill. Farther west are the heavily fortified Ukrainian strongholds of Kramatorsk and Sloviansk.

The ISW observed that Russia was also likely short of the mechanized forces it would need to push on from Bakhmut.

On Wednesday, Russian forces shelled scores of towns and villages in the Donetsk region and other areas in Ukraine’s east and south, Ukraine’s presidential office said.

In Kyiv, U.N. chief Guterres was discussing the possibility of extending the agreement that has kept at least some of the country's exports flowing.

Ukraine and Russia are leading global suppliers of wheat, sunflower oil and other agricultural products, and Moscow’s Feb. 24, 2022, invasion of Ukraine drove food prices higher across the world.

The current 120-day agreement expires on March 18, and Guterres said extending it for a second time is of “critical importance.”

“Exports of Ukrainian — as well as Russian — food and fertilizers are essential to global food security and food prices,” Guterres said.

In other developments Wednesday:

— The top intelligence official in the U.S. said American intelligence does not believe Russia can make “major territorial gains” in Ukraine this year because of heavy casualties and the Kremlin's inability to replenish weapons and ammunition. Speaking to a Senate committee, Avril Haines also cited other constraints on the Russian military, including dysfunction in leadership and declining troop morale.

— Germany's defense minister voiced caution over media reports that a pro-Ukraine group was involved in blowing up the Nord Stream gas pipelines in the Baltic Sea last year, as some media have reported. Boris Pistorius said more information is needed to understand who was behind the explosion — and whether they acted with the Ukrainian government's knowledge.

— Pistorius also said nearly two dozen advanced battle tanks provided by European countries will arrive in Ukraine in the coming weeks. He said that included 18 German Leopard 2 tanks and three from Portugal. The Leopards are part of a larger package of heavy tanks Western countries recently promised to Ukraine.

— A top European Union court has annulled the bloc’s sanctions against the mother of Wagner Group owner Prigozhin because the measures are based solely on the fact that the two are related. Violetta Prigozhina was put on the EU sanctions list because she was considered to be the owner of Concord Management and Consulting LLC, part of the group founded and owned until 2019 by her son. But the Luxembourg-based General Court said she stopped being owner of the company in 2017, even though she did retain some shares in it.

— Authorities in the Russian-held city of Enerhodar, which is the main residential area for workers at the Zaporizhzhia Nuclear Power Plant, said Ukrainian drones dropped incendiary devices along a river embankment, setting off fires that spread and endangered power lines.

The mayor of Enerhodar, Dmytro Orlov, said the smoke that arose in the city was caused by the burning of reeds along the embankment, but he did not say what caused the fire.