At least three civilians were killed and 16 others wounded Tuesday by Russian shelling in Kharkiv, Ukraine's second-largest city, according to the regional governor.

Kharkiv Oblast Gov. Oleg Sinegubov said Russian forces were using multiple-launch rocket systems on the northeastern city from a roughly 25-mile distance.

-ABC News' Iryna Hnatiuk

The Russian Ministry of Defense claimed Tuesday that a temporary cease-fire has been declared in the besieged city of Mariupol and that a humanitarian corridor has been opened to allow Ukrainian fighters to lay down their arms escape with their lives.

But Russian officials earlier stated that the Ukrainian fighters taking a stand at the Azovstal Iron and Steel Works plant in Mariupol had until 4 p.m. local time to surrender, a deadline that has now passed.

"For this purpose, a 'complete silence regime' has been introduced, any hostilities have been stopped, units of the Russian Armed Forces and the formations of the Donetsk People's Republic along the entire perimeter of Azovstal have been withdrawn to a safe distance," said Mikhail Mizintsev, head of the Russian National Defense Control Center.

Mizintsev said that the humanitarian corridor was opened "in view of the catastrophic situation at the Azovstal metallurgical plant in the city of Mariupol, and also guided by purely humane principles."

Ukrainian officials have not confirmed the opening of the humanitarian corridor out of Mariupol. Ukrainian Deputy Prime Minister Iryna Vereshchuk said earlier that humanitarian corridors would not be reopened on Tuesday.

Mizintsev said three humanitarian convoys have been set up in the immediate vicinity of the plant to take evacuees in three directions.

"Each humanitarian convoy includes 30 buses and vehicles for transporting people, 10 ambulances with medical and nursing teams," Mizintsev said. "In addition, meeting points and temporary accommodation have been deployed in all three directions, food points and primary medical care have been organized."

Russia is starting the next phase of its "special military operation" in neighboring Ukraine, according to Russian Foreign Minister Sergey Lavrov.

"This operation will continue. Another stage of this operation is beginning," Lavrov said in an interview Tuesday with English-language Indian television network India Today. "I am sure this will be a very important moment of this entire special operation."

Lavrov noted that the goal of the operation is to "fully liberate" the Russian-backed separatist regions of eastern Ukraine's Donetsk and Luhansk oblasts, "as it was declared from the very start."

Russia-backed separatist forces are trying to storm a steel plant in besieged Mariupol where Ukrainian troops are holed up, according to separatist spokesperson Eduard Basurin.

Basurin, spokesman of the militia for a pro-Russia breakaway republic of Donetsk in eastern Ukraine's disputed Donbas region, told Russian state media on Tuesday that a separatist special forces unit was chosen to assist the Russian military in storming the giant plant of the Azovstal Iron and Steel Works company in Mariupol, a strategic port city in eastern Ukraine's war-torn Donetsk Oblast that has been under heavy Russian bombardment since the start of the invasion on Feb. 24.

Basurin said they have already started their operation at the Azovstal plant, with Russian forces providing air and artillery support.

The territory of the Azovstal plant is the last holdout for the Ukrainian resistance in Mariupol, as Russian forces accelerate their efforts to capture city. The Mariupol City Council has previously said there are at least 1,000 people, including Ukrainian troops, on the grounds of the plant. Ukrainian Deputy Prime Minister Iryna Vereshchuk said Monday that civilians, including women and children, are also sheltering there and she called for an "urgent humanitarian corridor" to allow them to evacuate.

The Ukrainian government did not immediately confirm Tuesday that Russian forces are storming the Azovstal plant.

-ABC News' Yulia Drozd

Jalina Porter, the State Department's deputy spokesperson, is responding to the Russian attack at a Ukraine train station that killed at least 50, saying, "We can no longer be surprised by the Kremlin's repugnant disregard for human life."

Five children were among those killed when Russian rockets struck the station in Kramatorsk in Donetsk Oblast on Friday morning, according to Ukraine's state-owned railway company. At least 100 people were injured, according to Donetsk Oblast Gov. Pavlo Kyrylenko.

Russia has denied involvement in the attack, which occurred as "thousands" of civilians fleeing the Russian invasion were at the train station waiting to be taken to "safer regions of Ukraine," according to Kyrylenko.

"Civilians are killed when they stay in their homes, and they're killed when they try to leave," Porter said. "Actions like these demonstrate why Russia did not belong on the U.N. Human Rights Council, and they also reinforce the U.S. assessment that members of Russian forces are committing war crimes in Ukraine."

Porter declined to say if the department considers the train station attack a war crime, saying, "Assessing individual criminal liability in specific cases is the responsibility of courts, as well as other investigatory bodies. But as the secretary, Secretary [of State Antony] Blinken, has said, 'Those responsible for war crimes and other atrocities committed in Ukraine will be held to account.'"

-ABC News' Conor Finnegan