Zelenskiy Tells Russia 'This Is The Time To Meet'; Russian Troops Enter Mariupol.

Moscow-backed separatist troops in uniforms without insignia drive an armored vehicle in the besieged southern port city of Mariupol on March 19.

Soldiers, officials, and residents in the Ukrainian port city of Mariupol have desperately pleaded for Western help as invading Russian troops pushed deeper into the city center amid heavy fighting that has already shut down a massive steel plant.

The fighting came as Ukrainian President Volodymyr Zelenskiy called for fresh talks with Moscow more than three weeks into Russia's invasion of his country and as the Kremlin said it had for the first time in battle used hypersonic missiles in the west of Ukraine.

While the invasion has reportedly bogged down in much of the country and has resulted in heavy losses of troops and military equipment, Russian forces continue to bombard Ukrainian cities amid international condemnation and calls for an immediate cease-fire.

The Russian military has blasted major cities, including military and civilian sites alike, into rubble and has set siege to urban areas. British intelligence says an increasingly frustrated Kremlin has begun a strategy of attrition and is preparing for a extended battle.

In the strategic city of Mariupol, "children and the elderly are dying. The city is destroyed and it is wiped off the face of the earth," Mariupol police officer Mykhaylo Vershnin said in a video addressed to Western leaders and authenticated by the Associated Press.

Russia's heavy bombardment of the city, including a strike on a theater where hundreds of civilians were sheltering, has led to allegations that Russia was committing war crimes.

On March 18, Zelenskiy said that 130 people had been rescued from the theater but that "hundreds" more are still trapped under the rubble. No further details were available as of late on March 19.

Some 350,000 people remain inside the city in horrific conditions, aid workers say.

Vadym Denysenko, an adviser to Ukraine's interior minister, said in televised remarks that Ukrainian and Russian forces were on March 19 battling at Mariupol's Azovstal steel plant.

"One of the largest metallurgical plants in Europe is actually being destroyed," he said.

Russian forces appear intent on cutting the city off from the Sea of Azov and linking the Crimea Peninsula -- which was seized by Moscow in 2014 -- to territory controlled by Kremlin-backed separatists in eastern Ukraine.

Ukraine's Defense Ministry said it had "temporarily" lost access to the Sea of Azov as a result of Russian military operations.

In another Black Sea port city, Mykolayiv, local authorities and witnesses on March 19 reported that dozens of Ukrainian soldiers had been killed in a Russian air strike on a military barracks.

A 22-year-old Ukrainian soldier was quoted by AFP as saying that "no fewer than 200 soldiers were sleeping in the barracks" when Russian forces struck.

The regional governor also said an attack occurred but did not provide details. The reports could not immediately be independently confirmed.

Ukraine announced on March 19 that 10 humanitarian corridors had been set up with Moscow's agreement to allow civilians to escape fighting in cities around the country, including Mariupol, the capital, Kyiv, and for the evacuation of civilians in the eastern Luhansk region.

The open-source investigative group Bellingcat has published an interactive map of civilian facilities destroyed by Russia in the course of the war, based on video and photographic documenting several hundred incidents that have potentially harmed or affected civilians.

RFE/RL's Ukrainian Service reported that Russian troops were concentrating their shelling on eastern and southern Ukraine, and were regrouping near the capital, Kyiv.

Most regions sounded air alarms on the night of March 18-19, according to the service, but that shelling had become less intense in areas other than Mariupol, Mykolayiv, and areas adjacent to separatist-held parts of the eastern Luhansk and Donetsk regions.

Zelenskiy remained highly visible amid the invasion of his country.

In a video posted to Facebook on March 19, Zelenskiy accused Russia of attempting to destroy Ukraine and starve its cities into submission, but he called on Russian President Vladimir Putin to agree to direct talks to prevent unnecessary loss of life.

"This is the time to meet, to talk, time for renewing territorial integrity and fairness for Ukraine. Otherwise, Russia's losses will be such that several generations will not recover," he said.

Russia has provided only limited information on casualties, giving an early death toll of 498 soldiers, but Ukrainian and Western officials put the figure at several thousand, with several thousand more injured.

WATCH: A Ukrainian special search group collected the dead bodies of Russian soldiers in the hope they could be exchanged for Ukrainian prisoners of war.

Ukrainian casualty figures are also difficult to confirm.

In his video, shot on the streets of Kyiv, Zelenskiy denounced a huge March 18 rally in Moscow that Putin attended.

Zelenskiy said the estimated 100,000 people who reportedly gathered in front of Luzhniki stadium, along with the reported 95,000 who were inside the stadium itself, roughly corresponded to the number of Russian troops that had invaded Ukraine.

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Referring to Ukrainian estimates of Russian troop losses since the war began on February 24, Zelenskiy said, "And now imagine 14,000 corpses in this stadium, in addition to the tens of thousands more wounded and maimed people."

"The war must end, Ukraine's proposals are on the table," he added.

Russia's invasion of Ukraine, which began on February 24, has led millions of civilians to flee the country. The Polish Border Guard service has reported that more than 2 million refugees had entered Poland, while the United Nations has said more than 3 million have fled the fighting in Ukraine overall.

The World Health Organization said it had verified 43 attacks on hospitals and health facilities in Ukraine, killing 12 people and injuring 34.

The UN said at least 847 civilians have been killed and 1,399 wounded in Ukraine through March 18. The actual toll is thought to be much higher as the UN has not yet been able to verify casualty reports from several badly hit cities, it said.

On March 19, the Ukrainian Prosecutor-General's Office said 112 children had been killed and 140 wounded.

Most of the casualties in Ukraine have resulted from explosive weapons such as shelling from heavy artillery and multiple-launch rocket systems, and missile and air strikes, according to the UN human rights office.
Russia's military said it used its latest hypersonic missile for the first time in combat. It said Kinzhal missiles destroyed an underground site storing Ukrainian missiles and ammunition in the Ivano-Frankivsk region, which is less than 150 kilometers north of Romania and 250 kilometers from Hungary.

Russia has long boasted about its arsenal of Kinzhals, which are carried by MiG-31 fighter jets, have a range of up to 2,000 kilometers, and fly 10 times the speed of sound.

The Pentagon on March 19 said it could not confirm whether Russia had used a hypersonic missile in the attack.

With reporting by Reuters, AP, and AFP