Ukraine received fighter jets to help resist the Russian invasion, as Moscow intensified its offensive in the east where a besieged officer in Mariupol warned Wednesday his forces were facing their "last days, if not hours".
The West has responded to a renewed Russian push into the Donbas region with fresh weapons for Kyiv and a push to increase "Moscow's international isolation".
The Pentagon said that Ukraine had recently received fighter planes and parts to bolster its air force, declining to specify the number of aircraft and their origin.
Kyiv has asked its Western partners to provide MiG-29s, which its pilots already know how to fly and a handful of Eastern European countries have.
Control of Donbas and the southern port of Mariupol would allow Moscow to create a southern corridor to the Crimean peninsula that it annexed in 2014, depriving Ukraine of much of its coastline.
In the latest ultimatum issued in its battle to capture Mariupol, Moscow made another call for the city's defenders to surrender on Wednesday by 2 pm Moscow time (1100 GMT) and announced the opening of a humanitarian corridor for any Ukrainian troops who agreed to lay down their arms.
As the deadline approached, a commander in the besieged Azovstal power plant issued a desperate plea for help, saying his marines were "maybe facing our last days, if not hours".
"The enemy is outnumbering us 10 to one," Serhiy Volyna from the 36th Separate Marine Brigade said.
"We appeal and plead to all world leaders to help us. We ask them to use the procedure of extraction and take us to the territory of a third-party state."
An advisor to the mayor of Mariupol described a "horrible situation" in the encircled complex and reported that up to 2,000 people -- mostly women and children -- are without "normal" supplies of drinking water, food, and fresh air.
During an interview broadcast on CNN Tuesday, Pavlo Kyrylenko -- who oversees the Donetsk region's military administration -- insisted Mariupol remained contested.
"The Ukrainian flag is flying over the city," he said. "There are certain districts where street fighting is continuing. I can't say the Russians are controlling them."
Offering some respite, Kyiv said early Wednesday it had agreed with Russian forces to open a safe route for civilians to flee the devastated city.
"We have managed to get a preliminary agreement on a humanitarian corridor for women, children and elderly persons," Deputy Prime Minister Iryna Vereshchuk wrote on Telegram.
Elsewhere on the front lines, Ukraine's defence ministry reported its troops had beaten back a Russian attack in the city of Izium, south of the partly blockaded second city of Kharkiv.
In the town of Novodruzhesk, 65-year-old resident Nadya said: "We are bombed everywhere."
"It's a miracle that we're still alive," she said, her voice trembling.
"We were lying on the ground and waiting. Since February 24, we've been sleeping in the cellar."
The governor of the eastern Lugansk region Sergiy Gaiday said Ukrainian forces were holding their ground in the face of heavy fighting.
"We have positional battles in the cities of Rubizhne and Popasna. The enemy cannot do anything though. They are losing people and equipment there," Gaiday said.
"Our guys are shooting down drones there. Shooting down planes on the border of the Lugansk and Kharkiv regions, so they are holding on."
Russian forces, meanwhile, said "high-precision air-based missiles" hit 13 Ukrainian positions in parts of Donbas while other air strikes "hit 60 military assets", including in towns close to the eastern front line.
President Vladimir Putin has said he launched the so-called military operation in Ukraine in February to save Russian speakers in the country from a "genocide" carried out by a "neo-Nazi" regime.
But his forces have faced allegations of war crimes -- most recently from German Chancellor Olaf Scholz, who said Tuesday that Putin was responsible for atrocities in Ukraine.
"The killing of thousands of civilians as we have seen is a war crime for which the Russian president bears responsibility," Scholz said.
UN Secretary-General Antonio Guterres also denounced Russia's offensive, and issued calls for a four-day truce to mark the Orthodox Holy Week.
"Instead of a celebration of new life, this Easter coincides with a Russian offensive in eastern Ukraine," Guterres told reporters.
"The intense concentration of forces and firepower makes this battle inevitably more violent, bloody and destructive."
