Even as Russia and Ukraine intensified missile and drone strikes on each other’s major cities in recent days, their forces fought relentlessly along hundreds of miles of the frontline.

In the sprawling industrial city of Kostiantynivka, in Ukraine’s eastern Donetsk region, Ukrainian and Russian forces have been battling for months. In recent days, officials on each side have disputed who had control.

A spokesman for the Ukrainian military on Saturday denied an earlier Kremlin claim that Russian troops had taken the city, and said Ukraine was still holding it. The Ukrainian military also published a string of video messages from soldiers on Saturday, standing amid damaged buildings, saying they were defending the city. The messages could not be immediately verified.

Russian forces have targeted it for months, seeing it as the gateway to Kramatorsk, the last stronghold of Donetsk Province coveted by President Vladimir V. Putin of Russia but still held by Ukraine.

In what the Kremlin said was a visit to troops on Friday, Mr. Putin reiterated his determination to keep fighting in Ukraine and even to expand his sights to a wider area, although independent analysts say Russian troops are largely stalled in their advance, and at the cost of staggering casualties.

President Volodymyr Zelensky of Ukraine said on social media that he spoke by phone with President Trump on Saturday and agreed to continue talking in person during the NATO summit on Wednesday. Mr. Trump also spoke with Mr. Putin by telephone for nearly 90 minutes, Yuri Ushakov, a Kremlin aide, said in comments made public early on Sunday.

The fighting for Kostiantynivka has been slow and crushing. Russian forces have taken the area south of a river that dissects the town, and made advances along two prongs into the city.

As in previous eastern battles, like those for the cities of Bakhmut and Pokrovsk, the Russian bombardment has steadily destroyed buildings and positions, forcing Ukrainian units to retreat and allowing Russian soldiers to infiltrate the ruins.

Ukrainian troops were holding designated lines and had repelled 11 separate Russian assaults on Friday, Maj. Andriy Kovalov, spokesman for the Ukrainian General Staff, said in a statement on Saturday. “The situation remains difficult but is under the control of the Defense Forces of Ukraine,” he said.

Russian troops have been spotted in some parts of the city, including the city center, according to Brig. Gen. Oleksandr Bakulin, commander of the 19th Army corps, which is defending Kostiantynivka. More than 100 Russian soldiers had entered the city, he told a national news channel two weeks ago; some Ukrainian reporters have placed that figure closer to 250.

“At the moment, there is no specific area under the enemy’s control,” General Bakulin said. “There are individual buildings controlled by the enemy, and we are fighting that.”

The widespread use of attack drones by both sides has made movement in and around the city perilous. Unlike in previous battles, Russian soldiers were not infiltrating in units or even in pairs, but one at a time, General Bakulin said.

Ukraine did not have the troops to prevent Russian forces from reaching the city, and in places, had made tactical retreats. “We did not try to hold every position at any cost by sacrificing large numbers of people,” he said.

The situation in Kostiantynivka is mirrored in many places along the frontline in Donetsk. Despite Ukraine’s recent success with midrange drone attacks that have struck Russian supply lines in rear positions in southern Ukraine, Ukrainian soldiers in Donetsk still complain of a lack of manpower, ammunition and drones.

And Ukraine has still been unable to stem devastating Russian raids using aerial-guided bombs, which have smashed towns and cities in the path of the Russian advance. A band of cities — Sloviansk, Kramatorsk, Druzhkivka and Kostiantynivka — have been badly damaged by the aerial bombing in recent months, causing thousands of civilians to flee.

Russian planes unleashed six guided bombs on the city of Sumy in northeastern Ukraine on Friday, local officials said. One killed four people in the city center, including a 5-year-old girl and her mother.

In southern Ukraine, Russian forces turned to attack thousands of acres of farmland and farm equipment with a swarm of drones in the last week, according to a local farmer, Viktor Hordiienko.

“The attacks on our fields began in 2024, but they were nothing like what we are experiencing now,” Mr. Hordiienko said by telephone.

“Last autumn, during the sowing season, my team and I came under 29 separate drone attacks,” he said. “But now the situation has become overwhelming. We are seeing up to 100 Russian drones a day in our area. It is simply impossible to defend ourselves against that number.”

Russian positions in the southern Kherson region were 30 miles away from his fields, he said. The drones, which he said were dropping explosives or detonating on impact, were targeting his fields and setting the dry wheat alight just as it was ready to harvest.

“At this point, I honestly do not believe we will be able to harvest our crop,” he said.

Ivan Nechepurenko contributed reporting.