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 besieged 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 issued 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."

Thousands of troops and civilians remain holed up in the plant.

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.

But 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."

- 'We are bombed everywhere' -

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."

Kyiv also claimed enemy losses in a Ukrainian counter-attack near the town of Marinka in Donetsk.

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.

- 'War crime' - 

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," Scholtz said.

UN Secretary-General Antonio Guterres also denounced Russia's ongoing 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."

Guterres for a "humanitarian pause" from Holy Thursday until Easter Sunday on April 24.

"Hundreds of thousands of lives hang in the balance."

Latest developments

Here are the latest developments in the war in Ukraine:

- Donbas offensive -

Russia says its forces have carried out dozens of air strikes in eastern Ukraine as part of a new offensive in the Donbas region that had been expected for days.

Moscow says the strikes targeted Ukrainian positions in parts of the Donbas as well as in towns close to the eastern frontline. 

"We are gradually implementing our plan to liberate the Donetsk and Lugansk People's Republics," Defence Minister Sergei Shoigu says, referring to eastern Ukraine's two rebel regions, which Moscow has recognised as independent states.

- Fighting in Mariupol -

Fighting between Ukrainian and Russian forces is "ongoing" in the strategic port of Mariupol, Pavlo Kyrylenko, governor of Ukraine's eastern Donetsk region, says.

Russia earlier on Tuesday called on Ukrainian forces in Mariupol to "immediately" lay down their arms in a new ultimatum.

Russian forces are believed to have gradually pushed their way into the city, pinning Ukrainian troops down to giant factories near the port, which are equipped with vast underground tunnels.

- Biden talks with allies -

US President Joe Biden holds a virtual meeting of allies to discuss the Ukraine conflict.

The videoconference brings together the leaders of France, Britain, Germany, Romania, Poland, Italy, Canada and Japan.

The United States and the European Union have reached a "broad consensus" on the need to increase pressure on Russia through new sanctions, the Italian government says after the talks.

German Chancellor Olaf Scholz says after the meeting Russian President Vladimir Putin is responsible for war crimes in Ukraine that have already left thousands of civilians dead.

- Easter truce call -

UN Secretary-General Antonio Guterres denounces Russia's fresh offensive in eastern Ukraine and calls for a four-day truce to mark Orthodox Holy Week.

"Hundreds of thousands of lives hang in the balance," the United Nations chief says, adding a truce would allow safe passage for civilians to flee the eastern war zone, and for humanitarian aid to be delivered to the hardest-hit areas.

- New US arms delivery -

The first shipments of a new US military aid package have arrived at Ukraine's borders to be handed over, a senior Pentagon official says.

The package includes 18 155mm howitzers for the first time, as well as 40,000 artillery rounds, 200 M113 armoured personnel carriers, 11 Mi-17 helicopters and 100 armoured multi-purpose vehicles.

- Growth hit -

The "seismic" impact of the war in Ukraine is spreading worldwide, causing the IMF to sharply downgrade its 2022 global growth forecast to 3.6 percent, 0.8 points lower than in January.

The latest World Economic Outlook shows Ukraine suffering a 35 percent collapse this year, while Russia sees a drop of 8.5 percent.

- Diplomats expelled -

Russia says it is expelling 36 diplomats from Belgium and the Netherlands in retaliation for similar measures taken against its envoys in the two EU countries.

- Nearly 5 mn have fled Ukraine: UN -

More than 4.9 million Ukrainians have fled their country following the Russian invasion, says the UN's refugee agency.

the UNHCR says 4,980,589 Ukrainians had left since Russia invaded on February 24 -- a figure up 46,174 on Monday's total.