Russian Defense Minister Sergei Shoigu has accused the United States and other Western countries of doing everything in their power to drag out Russia's so-called special military operation in neighboring Ukraine.

"The increasing volume of foreign arms supplies clearly demonstrates their intentions to provoke the Kyiv regime to fight to the last Ukrainian standing," Shoigu told officials during a televised meeting Tuesday.

Shoigu said it is important Russia continue improving its armed forces in light of recent events in Ukraine.

Shoigu added that the Russian army is "fulling the tasks set by the commander-in-chief." He said a new offensive in eastern Ukraine's disputed Donbas region is "being consistently implemented, and measure are being taken to establish a peaceful life."

-ABC News Christina Theodorou

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Armed Forces and the formations of the Donetsk People's Republic along the entire perimeter of Azovstal have been withdrawn to a safe distance," said Mikhail Mizintsev, head of the Russian National Defense Control Center.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."

Mariupol has not yet fallen, despite Russia's demands that Ukrainian troops defending the besieged Ukrainian port city surrender, according to Ukrainian Prime Minister Denys Shmyhal.

"There [are] still our military forces, our soldiers, so they will fight until the end," Shmyhal told ABC News' George Stephanopoulos in an exclusive interview Sunday on "This Week."

Mariupol is a strategic city for Moscow because it would allow Russian forces in the south to connect with troops in eastern Ukraine's contested Donbas region. It would also give Moscow a key port.

Although Mariupol remains under the Ukrainian government's control, Shmyhal said the city's residents are suffering.

"They have no water, no food, no heat, no electricity," he said. "They ask all of our partners to support and help stop this humanitarian catastrophe."

-ABC News' Monica Dunn