Russian Forces Are Doing ‘Everything They Can’ to Stop Counteroffensive, Zelensky Says

President Volodymyr Zelensky of Ukraine said that Russian forces were throwing “everything they can” at Kyiv’s troops fighting to retake land in the south and east, again emphasizing the grueling nature of a counteroffensive that is moving slower than some allies had hoped.

Ukrainian troops have made only small gains since launching the widely anticipated campaign in June, and in recent weeks, they appear to have stalled in some areas in the face of staunch Russian defenses. Casualties are mounting, and American officials have said that Ukraine has also lost newly provided Western armored vehicles in field after field of land mines.

Mr. Zelensky, who has defended the pace of the counteroffensive, said in his overnight address late Friday that he had had a “detailed” meeting earlier in the day with his top commanders to discuss the front lines and “logistics” — including weapons and the “rational use of shells, supplies from partners,” an apparent reference to the rate at which Ukraine’s forces are expending ammunition.

“We must all understand very clearly — as clearly as possible — that the Russian forces on our southern and eastern lands are investing everything they can to stop our warriors,” he said. “Every thousand meters of advance, every success of each of our combat brigades deserves gratitude.”

Mr. Zelensky has repeatedly pressed his Western allies for increasingly sophisticated weapons, and he secured new pledges this week from allies at the NATO summit in Lithuania, including long-range missiles from France and more tank ammunition from Germany. But it was not immediately clear how soon those weapons would arrive, or how significant a boost they could provide for the counteroffensive.

The United States has acknowledged that Ukrainian forces are running low on ammunition, which was one reason that President Biden agreed last week over the objections of allies to send cluster munitions to Ukraine. The weapons are highly dangerous for civilians and are outlawed by all but a few countries, including the United States, Russia and Ukraine.

While the cluster munitions have started arriving in Ukraine, American officials and military analysts have warned that they probably will not be an immediate help.

Ukraine’s top commander, Gen. Valeriy Zaluzhnyi, told The Washington Post in an interview published on Friday that his military was still lacking the necessary resources to defeat Russia and criticized allies who have argued that it does not need F-16s.

The defense ministers of Denmark and the Netherlands announced this past week that they had gathered 11 countries to help train Ukrainian pilots on F-16 fighter jets as soon as next month. Mr. Biden agreed in May to drop his objections to giving Ukraine F-16s, though that may not happen until next year.

Ukraine has also been asking the United States for long-range Army Tactical Missile Systems,which have a range of about 190 miles — about 40 miles more than missiles that France and Britain are providing. American and European officials have said that the Biden administration, after months of maintaining it would not provide the weapons for fear of further provoking Russia, is considering whether to send a few to Ukraine.

One ally that has resisted sending weapons to Ukraine is South Korea, whose president, Yoon Suk Yeol, arrived in Ukraine on Saturday on an unannounced trip.

Seoul, which is reluctant to openly antagonize Moscow, has declined to send lethal aid and has imposed strict export control rules on its global weapons sales. It has also provided humanitarian aid and financial support to Ukraine for mine removal, power grid restoration and reconstruction projects.

However, Mr. Yoon has indicated that Seoul might be willing to consider sending Ukraine military aid in the event of a large-scale attack on civilians.

He visited the towns of Bucha and Irpin — which became synonymous with Russian atrocities in the earliest days of the invasion — upon arrival on Saturday, and was later expected to meet with Mr. Zelensky, Mr. Yoon’s office said.

In the meantime, Ukraine’s military on Saturday continued to report fierce fighting in the country’s south and east, saying that Russian forces in southern Ukraine were focused on “preventing the further advance” of Kyiv’s troops fighting in the direction of two key Russian-occupied cities, Melitopol and Berdiansk.

John Yoon contributed reporting.