KYIV, Ukraine — Hundreds more Ukrainian fighters who made their stand inside Mariupol’s bombed-out steel plant surrendered, bringing the total to over 1,700, Russia said Thursday, amid international fears the Kremlin will take reprisals against the prisoners.

The Red Cross registered hundreds of the soldiers as prisoners of war in a step toward ensuring their humane treatment under the Geneva Conventions.

Meanwhile, in the first war crimes trial held by Ukraine, a captured Russian soldier testified that he shot an unarmed civilian in the head on an officer’s orders, and he asked the victim’s widow to forgive him. The soldier pleaded guilty earlier in the week, but prosecutors presented the evidence against him in line with Ukrainian law.

In Mariupol, the nearly threemonth siege that turned the strategic port city into a symbol of the war’s horrors drew ever closer to an end as the fighters in the last bastion of resistance continued abandoning the Azovstal steel plant on orders from above to save their lives.

The Russian military said 1,730 Ukrainian troops at the steelworks have surrendered since Monday. At least some were taken by the Russians to a former penal colony in territory controlled by Moscow-backed separatists. A separatist official said others were hospitalized.

It was not clear how many fighters were left in the maze of tunnels and bunkers at the plant. Russia in recent weeks had estimated that it had been battling some 2,000 troops at the steelworks.

The International Committee of the Red Cross said it registered hundreds of POWs from the plant under an agreement between Russia and Ukraine. It did not say whether it had visited the prisoners.

While Ukraine said it hopes to get the soldiers back in a prisoner swap, Russian authorities have threatened to investigate some for war crimes and put them on trial, branding them “Nazis” and criminals.

The defense of the steel mill has been led by Ukraine’s Azov Regiment, whose far-right origins have been seized on by the Kremlin as part of an effort to cast its invasion as a battle against Nazi influence in Ukraine.

Those threats and accusations have raised fears about the fate of the captured fighters.

Amnesty International had pushed for the Red Cross to be given access to the troops, citing lawless executions allegedly carried out by Russian forces in Ukraine and saying the Azovstal defenders “must not meet the same fate.”

The emptying of the plant would allow Russia to claim complete control of Mariupol, a long-sought victory but one that holds largely symbolic importance at this point since the city is already effectively under Moscow’s control and military analysts say most of the Russian forces that were tied down by the drawn-out fighting have already left.

Still, it would be a clear win in a war that has seen Moscow suffer a series of setbacks in the face of unexpectedly stiff Ukrainian resistance. Kyiv’s troops, bolstered by Western weapons, thwarted Russia’s initial goal of storming the capital and have tied down Moscow’s forces in the Donbas, the eastern industrial region that President Vladimir Putin has set his sights on capturing.

The surprising success of Ukraine’s troops has buoyed Kyiv’s confidence, and a senior official reflected that Thursday.

Mykhailo Podolyak, an adviser to Ukrainian President Volodymyr Zelenskyy who was involved in several rounds of talks with Russia, said in a tweet addressed to Moscow: “Do not offer us a cease-fire — this is impossible without total Russian troops withdrawal.”

“Until Russia is ready to fully liberate occupied territories, our negotiating team is weapons, sanctions and money,” he wrote.

Russia, though, signaled its intent to incorporate or at least maintain influence over areas its forces have seized.

In other developments, Gen. Mark Milley, chairman of the U.S. Joint Chiefs of Staff, spoke by phone on Thursday with his Russian counterpart for the first time since the war began, and they agreed to keep the lines of communications open, the Pentagon said.

On the battlefield, Ukraine’s military said Russian forces pressed their offensive in various sections of the front in the Donbas but were being repelled. The governor of the Luhansk region said Russian shelling killed four civilians, while separatist authorities in Donetsk said Ukrainian shelling killed two.