Ukraine and Russia have exchanged the bodies of 320 fallen soldiers in the Zaporizhzhia region.

The operation, which took place Thursday, followed a negotiated agreement between the warring sides to transfer the remains of soldiers on a one-to-one basis, according to a statement released Saturday by Ukrainian officials.

Previously, Ukrainian officials had said Russia had been reluctant to discuss repatriating its dead. Ukrainian soldiers have been tasked with recovering the bodies of Russian soldiers and placing them in refrigerated railway cars in several cities, including Kyiv, the capital, as well as Zaporizhzhia and Dnipro.

After the Ukrainian army successfully counterattacked Russian forces and pushed them farther away from Kharkiv, Ukraine’s second-largest city, two soldiers began gathering remains in the area. As residents returned to Kharkiv and surrounding villages, some have found the bodies in their homes or have stumbled across them elsewhere.

The two soldiers running the recovery, who both declined to be identified, said many of the bodies had been lying in the open for a month or longer before they found them. The pair works to identify the soldiers by their faces, tattoos and belongings. They also take a DNA swab from each corpse to determine whether any soldiers believed to be involved in war crimes are among them.

One of the soldiers on the recovery duty said identifications were possible about half of the time, with the remainder of the corpses being too deteriorated.

Russia has not released casualty figures for its troops since late March, when it said 1,351 soldiers had died. Ukrainian President Volodymyr Zelenskyy said his officials believe that at least 30,000 Russian troops have been killed.

Recently, a British intelligence assessment put the estimated Russian losses at half that number.

Ukraine, similarly, has not disclosed data on its military casualties, but Zelenskyy has said as many as 100 service members might be dying daily in the fighting in the eastern Donbas region. U.S. intelligence agencies estimated in mid-April that between 5,500 and 11,000 Ukrainian soldiers had been killed.