Russia's defense ministry issued a new proposal to Ukrainian troops holed up in the Azovstal steel plant to lay down their weapons on Wednesday, adding that not a single Ukrainian soldier had accepted that same offer on Tuesday.
Russian troops will observe a ceasefire in the area of Azovstal while the proposal is in effect starting from 1400 Moscow time (1100 GMT) on April 20, the ministry added in a statement.
A Ukrainian presidential adviser said on Tuesday that Russia was hitting Azovstal, the main remaining Ukrainian stronghold in the city of Mariupol, with bunker-buster bombs.
"The world watches the murder of children online and remains silent," adviser Mykhailo Podolyak wrote on Twitter.
Mariupol's city council said on Monday that over 1,000 civilians were sheltering in basement premises underneath the Azovstal plant.
Russia has denied targeting civilians.
Four people were killed in Ukraine's eastern city of Kharkiv on Tuesday, according to reports by local officials.
Another 14 people were also wounded by Russian rockets in the city on Tuesday.
"More than 10 residential buildings were damaged, as well as garages and a supermarket," the regional prosecutor's office wrote on Facebook.
Russia is advancing across almost the entire stretch of the eastern front in its war on Ukraine. Ukrainian officials said their soldiers would withstand the offensive.
As the 55th day of the Russia-Ukraine War dawned, the Russian military was still preparing their forces for an expected major offensive in the Donbas region of Ukraine, according to US defense officials, but Ukrainian officials have also said that the Battle of Donbas has already begun.
Battle of Donbas
"We have seen the Russians continue to flow in enablers, capabilities that will help them fight in the Donbas going forward, that's artillery, rotary aviation, helicopter support, command and control enablers," said Pentagon Press Secretary John Kirby on Tuesday morning. "And we do believe that they have reinforced the number of battalion tactical groups in the east and the south of Ukraine."
A senior Defense Department official said that an additional 11 battalions had been added to the Russian force in Ukraine, for a total of 76.
"It's difficult to say that that's some sort of sign that, you know, that there's an imminence here," said the DoD official. "We still believe that the Russians are doing a fair amount of shaping."
Ukrainian official sources also confirmed the reenforcement of Donbas region's Russian forces. Ukrainian Armed Forces Commander-in-Chief General Valery Zaluzhny said that Russia had regrouped its forces in recent weeks.
The Ukrainian military said on Tuesday morning that combat units, support units, weapons, and equipment from central and east Russia were being sent to the Donbas front. This also includes several surface-to-air missile defense systems, such as the S-300 and S-400 weapon platforms, to the border region north of Kharkiv.
However, Ukrainian officials have said that the Battle of Donbas has already begun.
"It can now be stated that Russian troops have begun the battle for Donbas, for which they have been preparing for a long time. A very large part of the entire Russian army is now focused on this offensive," Ukrainian President Volodymyr Zelensky said late Monday night.
The Ukrainian military said early Tuesday morning that the objective of Russian operations in the east was to establish full control of the separatist Donetsk and Luhansk provinces, and to create a land corridor with Crimea.
Zelensky said that Russian forces were probing Ukrainian defenses in the south and the east of Ukraine. Zaluzhny said that Russia was concentrating the main efforts on Izium, which is between Kharkiv, and Donetsk and Luhansk. Kharkiv and Mariupol were still bearing the brunt of Russian bombing, and Mariupol was still contested ground, said the DoD official. The official also noted that should Mariupol fall, it would free up almost a dozen battalions for fighting elsewhere.
Russia-backed separatist forces are trying to storm the Azovstal metallurgical plant in the besieged Ukrainian port of Mariupol, the RIA news agency quoted separatist spokesperson Eduard Basurin as saying on Tuesday.
Another separatist, Denis Pushilin, was also quoted by RIA, explaining that the separatists aim to "liberate" the facility as quickly as possible.
Intensifying missile attacks
Russian Missile attacks did not stop throughout the night and have increased in intensity in striking west Ukraine locations, the Ukrainian military said on Tuesday morning.
A US Defense Department senior official said that Russia has launched over 660 missile strikes since the war began. In the last day, Russia conducted more than 200 air sorties, including by long-range bombers firing cruise missiles.
"The Russian army is not slowing down the use of missiles against Ukraine," said Zelensky in a Monday night address, echoing an earlier statement by Zaluzhny that "the danger of missile and bomb strikes remains for the whole country." Zaluzhny cautioned citizens to heed warning sirens and to be aware of bomb shelters.
The Russian Defense Ministry said on Tuesday morning that it had struck a logistics center near Lviv, an ammunition depot near Kyiv, and 331 other military sites using ballistic missiles and artillery. The ministry also claimed to have destroyed a large shipment of American and European weapons that had arrived in Ukraine, but in a Tuesday morning statement, a senior US Defense Department official denied that they were aware of any such shipment being destroyed.
Despite the alleged damage done by the strikes, Zelensky noted on Monday that "none of the missile strikes has changed the situation for Russia in any fundamental way. And if you evaluate them all together, the conclusion is strategic nonsense."
Naval threat not sunk
Zaluzhny said that the sinking of the Russian Black Sea Fleet flagship, the Moskva, significantly reduced Russia's naval capabilities. A US DoD official said that the Russian Navy was "maintaining a standoff distance from Odesa," in the wake of the sinking, which Ukraine claims were the result of twin Neptune anti-ship missiles.
Zaluzhny warned that the danger of amphibious and paratrooper landings remained. In response, the Ukrainian military was trying to shore up coastal defenses.
A US DoD official said that the US couldn't independently confirm the preparations for an amphibious assault. There was at least one tank landing ship in the Sea of Azov, and that "they certainly have others in the Black Sea."
The Ukrainian military claimed that the Black Sea Fleet has seen 158 servicemen killed, 500 wounded, and 70 missing. It also claimed that some sailors were refusing to participate in the war.
Rising casualties
Since the war began, the Russian Defense Ministry has claimed to have destroyed 139 warplanes, 483 drones, 250 anti-air systems, 2,337 tanks and other ground vehicles, 254 multiple launch rocket systems (MLRS), 1009 artillery pieces, and 2196 other military vehicles.