LVIV/KYIV (AFP, REUTERS) - Russian Foreign Minister Sergei Lavrov said on Tuesday (April 19) that Moscow was starting a new stage of what it calls its special military operation in Ukraine which he predicted would be a significant development.

“Another stage of this operation (in eastern Ukraine) is beginning and I am sure this will be a very important moment of this entire special operation”, Lavrov said in an interview with the India Today TV channel.

The Russian defence ministry said earlier its forces carried out dozens of air strikes in eastern Ukraine overnight.

The ministry said that “high-precision air-based missiles” had hit 13 Ukrainian positions in parts of the Donbas, including the key town of Slovyansk, and that other air strikes “hit 60 military assets of Ukraine”, including in towns close to the eastern frontline.

It said Russian troops destroyed two warehouses containing warheads of Tochka-U tactical missiles in Chervona Polyana, in the region of Luhansk, and in Balakliia in the Kharkiv region.

A total of 1,260 military targets were hit by rockets and artillery overnight, the ministry said in a statement.

“Russian air defence systems shot down a Ukrainian MiG-29 fighter near the village of Malynivka in the Donetsk region,” the statement added.

Earlier on Tuesday, Ukrainian President Volodymyr Zelensky said Russia had begun the "Battle of Donbas" in the east on Monday and a "very large part of the entire Russian army is now focused on this offensive".

"No matter how many Russian troops they send there, we will fight. We will defend ourselves," he vowed in a video address.  

Zelensky’s chief of staff Andriy Yermak called it "the second phase of the war" and assured Ukrainians that their forces could hold off the offensive.

"Believe in our army, it is very strong," he said.  

Ukrainian media reported a series of explosions, some powerful, along the front line in the Donetsk region, with shelling taking place in Marinka, Slavyansk and Kramatorsk. 

Ukrainian local officials and local media also said explosions were heard in Kharkiv in the northeast of Ukraine, Mykolaiv in the south and Zaporizhzhia in the south-east.  

Reuters was not immediately able to verify the reports.  

In the besieged city of Mariupol, Russia-backed separatist forces are trying to storm the Azovstal metallurgical plant, the RIA news agency quoted separatist spokesperson Eduard Basurin as saying on Tuesday.

The separatists aim to “liberate” the facility as quickly as possible, RIA quoted another separatist, Denis Pushilin, as saying.

Ukrainian officials said Russian shelling killed four people in the Donetsk region on Monday, while a man and a woman were killed in Kharkiv when shells hit a playground near a residential building.  

They said a Russian missile attack also killed seven people in Lviv, the first civilian victims in the western city about 60km from Poland. 

Ukraine’s top security official, Oleksiy Danilov, said Russian forces attempted to break through Ukrainian defences "along almost the entire front line of Donetsk, Luhansk and Kharkiv regions" on Monday morning.  

Driven back by Ukrainian resistance in the north, Moscow has refocused its ground offensive in the two eastern provinces known as the Donbas, while launching long-distance strikes at other targets including the capital, Kyiv.  

Donbas has been the focal point of Russia’s campaign to destabilise Ukraine, starting in 2014 when the Kremlin used proxies to set up two separatist "people’s republics" in the ex-Soviet state.

It is also home to much of Ukraine’s industrial wealth, including coal and steel.  

Russia’s defence ministry said it had hit hundreds of military targets in Ukraine overnight. It said air-launched missiles had destroyed 16 military facilities in the Kharkiv, Zaporizhzhia, Donetsk and Dnipropetrovsk regions and in the port of Mykolayiv, which are in south and east Ukraine.  

It added that the Russian air force had launched strikes against 108 areas where Ukrainian forces were concentrated and Russian artillery struck 315 Ukrainian military targets. 

Biden to host call with allies

Western capitals and Kyiv accuse Russian President Vladimir Putin of unprovoked aggression and the White House said US President Joe Biden would hold a call with allies on Tuesday to discuss the Ukraine crisis, including on how to coordinate on holding Russia accountable.

The United Nations said on Monday the war’s civilian death toll had surpassed 2,000, reaching 2,072 as of midnight on April 17 from the start of the Russian invasion on Feb 24. 

 About 4 million Ukrainians have fled the country.

Russia denies targeting civilians in what it calls a special operation to demilitarise Ukraine and eradicate dangerous nationalists. It rejects what Ukraine says is evidence of atrocities, saying Ukraine has staged them to undermine peace talks.

Last week, Biden announced an additional US$800 million in military assistance to Ukraine, expanding the aid to include heavy artillery ahead of a wider Russian assault expected in eastern Ukraine.

The US military expects to start training Ukrainians on using howitzer artillery in coming days, a senior US defence official said on Monday, adding that the training would take place outside Ukraine.

French President Emmanuel Macron said on Monday his dialogue with Putin had stalled after mass killings were discovered in Ukraine.

'Hell on Earth'

Russia has been trying to take full control of the southeastern port city of Mariupol, which has been besieged for weeks and which would be a huge strategic prize, linking territory held by pro-Russian separatists in the east with the Crimea region that Moscow annexed in 2014.

Video footage showed block after residential block in charred ruins. Shell-shocked residents in the Primorskyi district cooked on open fires outside their damaged homes.

"To be honest, we are not well," one resident named Olga told Reuters. "I have mental problems after airstrikes, that’s for sure. I’m really scared. When I hear a plane I just run away."

The city council said at least 1,000 civilians were still hiding in underground shelters beneath the vast Azovstal steel plant, which contain myriad buildings, blast furnaces and rail tracks.

Major Serhiy Volyna, commander of Ukraine's 36th marine brigade which is still fighting in Mariupol, appealed for help in a letter to Pope Francis, saying women and children were trapped among fighters in the city's steel works.

"This is what hell looks like on earth ... It's time (for) help not just by prayers. Save our lives from satanic hands," the letter said, according to excerpts tweeted by Ukraine's ambassador to the Vatican.

Azovstal is the main remaining stronghold of Ukrainian forces in Mariupol. The city's defenders include Ukrainian marines, motorised brigades, a National Guard brigade and the Azov Regiment, a militia created by far-right nationalists that was later incorporated into the National Guard.

Denis Prokopenko, a lieutenant colonel of the Azov Regiment, said in a Telegram video post that while he was at the steel plant, Russian and separatist forces were dropping anti-bunker bombs and blasting the area with rockets and other weapons, including from ships, "knowing that there are civilians here."

Russia's invasion has damaged or destroyed up to 30 per cent of Ukraine's infrastructure at a cost of US$100 billion, a Ukrainian minister said, adding reconstruction could be achieved in two years using frozen Russian assets to help finance it.