DONETSK, Ukraine — The masked separatist fighter standing outside the morgue of Kalinina hospital was visibly agitated as he clutched his automatic rifle and recalled how 30 of his colleagues died earlier on Tuesday morning after hours of battling the Ukrainian military. “They were the wounded being taken off the battlefield when the fascists fired down on their truck,” said the man, who refused to give his name or take off the ski mask hiding his face. His claims could not be independently verified. Whatever the true details of the fighting, there was little doubt of the cost. In the morgue behind him, there were piles of dead fighters in fatigues on the floor, sometimes three on top of one another. Some were missing limbs. More corpses were on the examination tables in the main room of the morgue, where workers said they were past capacity and could take no more bodies. “We will seek revenge all the way to Kiev and then take that too,” said the masked man, in a sleeveless black T-shirt and camouflage pants. “The Donbas will never forgive this action.” The Donbas region spans parts of eastern Ukraine and territory in Russia, and Donetsk is its main city in Ukraine. Pro-Russian separatists estimated that they lost as many as 50 men in heavy fighting with Ukrainian forces in the last 24 hours as the two sides battled for control over the regional airport in Donetsk. The mayor, Oleksandr Lukyanchenko, estimated that those losses were closer to 40, including two civilians caught in crossfire.

The fighting started Monday afternoon, after rebels tried to take the important regional airport in the early morning, just hours after polls closed in Ukraine’s May 25 presidential election. The Ukrainian military gave them an ultimatum to surrender the airport by noon. When the rebels refused, the Ukrainian military went on the offensive, with airstrikes and fierce ground fighting. Fighting continued throughout the night, and the sound of large explosions and shooting could be heard from the center of Donetsk, a city of 1 million people. By midday Tuesday, the Ukrainian government said it had regained control of the airport, which remained closed and all flights canceled. “The enemy suffered heavy losses. We have none, ” Ukraine’s Interior Minister Arsen Avakov said in a statement. The fighting that started Monday seemed to be a surge of intensity for what the Ukrainian government called an “anti-terrorist” operation, which began more than a month ago and has had mixed results against the heavily armed pro-Russian rebels. President-elect Petro Poroshenko, who won more than 54 percent of the vote on Sunday, promised in his election night victory speech to prioritize visiting the Donbas and opening a dialogue with the eastern half of the country. He said he would negotiate only with separatists willing to lay down their weapons, saying, “No country in the world would negotiate with terrorists.” Poroshenko’s words fell on deaf ears in Donetsk, which has now seen the worst violence since the country’s independence from the Soviet Union in 1991. Many eastern Ukrainians dismissed Sunday’s presidential election, which was seen by many other parts of the country as a step toward re-establishing legitimate power in the deeply divided country. “These are the new Ukrainian president’s welcoming words to the Donbas?” said Leonid Baranov, a rebel fighter who escorted journalists into the morgue, instructing them not to take photos of the dead men’s faces. “If this is his democracy, we don’t want anything to do with it.” A produce truck parked outside the Soviet-era brick building on Tuesday delivered the dead to the morgue, and its back compartment was wet with large pools of blood. Before taking their bodies inside, the rebels removed personal belongings and laid them out on the pavement in front of the entrance to the morgue. Among the half-empty cigarette boxes and camouflage green canteens stained with blood were first-aid kits with German labels and Orthodox crosses on beaded chains. Baranov said that there were at least three other trucks full of dead rebels who were delivered to other morgues in the city, wherever there was space.