Ukrainian President Volodymyr Zelensky says a large-scale military operation by the Russian military in the country’s east has started.

Zelensky says Russian forces had begun the "Battle of Donbass" after senior officials said Moscow had begun a new offensive push along most of Ukraine's eastern flank.

"We can now confirm that Russian troops have begun the battle for the Donbass, which they have been preparing for a long time. A large part of the Russian army is now dedicated to this offensive," Zelensky said in a video address, adding: "No matter how many Russian troops they send there, we will fight. We will defend ourselves."

Russian Foreign Minister Sergey Lavrov confirmed, “another stage of this operation is beginning… and I am sure this will be a very important moment of this entire special undertaking.”

Ukrainian media reported a series of explosions, some powerful, along the front line in the Donetsk region in Donbass. 

Ukrainian officials and local media also say further explosions were heard in Kharkiv, Mykolaiv in the south, and Zaporizhzhia in the southeast.

Tehran Times was not immediately able to verify the reports. 

The latest developments come as the governor of Lviv, said preliminary reports suggest there have been four strikes in the city, three on warehouses and another on a car service station.

The Russian defense ministry says the country’s air forces have carried out airstrikes at a logistics center of the Ukrainian army near the city of Lviv and destroyed a large number of foreign-made weapons stored there.

The Russian forces have also destroyed a repair center for Tochka U ballistic missiles in the city of Dnipro, the ministry says.

Local authorities say Russian troops captured the eastern Ukraine town of Kreminna, as Kyiv's armed forces launched salvos on Russian forces in the nearby settlement of Rubizhne. Kreminna is said to be a strategic target for Moscow.

Rubizhne, which is under control of Russian forces, was under intense fire from Ukrainian artillery and mortars, journalists reported.

Powerful explosions could be seen in Rubizhne, sometimes followed by fires and plumes of white or black smoke.

Russian forces do appear to have stepped up their operation to capture the eastern Donbas region having pulled back troops deployed around the capital Kyiv.

Russia's defense ministry also says it had hit hundreds of military targets in Ukraine. It says air-launched missiles had destroyed 16 military facilities across 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.

A senior Pentagon official says the first shipments of a new U.S. military aid package have arrived at Ukraine's borders to be handed over in its fight against the Russian military,

The United States April 13 unveiled an $800-million tranche of equipment for Ukraine, including helicopters, howitzers, and armored personnel carriers.

"There have been four flights from the United States arriving into the theater just yesterday," a senior Pentagon official says, with a fifth flight due shortly.

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

Last week, U.S. President Joe Biden announced an additional $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 U.S. military expects to start training Ukrainians on using howitzer artillery in the coming days, a senior U.S. defense official says, adding the training would take place outside Ukraine.

Russia has formally complained to the U.S. over its military aid to Ukraine, warning of "unpredictable consequences" if shipments of advanced weaponry go forward. 

Meanwhile, UN humanitarian chief Martin Griffiths has announced he will push for the creation of a "humanitarian contact group" involving Ukraine and Russia. 

Such a group would "meet convened by the UN to discuss on a virtual or actual basis at any time to discuss humanitarian issues," Griffiths told reporters in New York.

That could include ceasefire monitoring, safe passage, humanitarian corridors or other issues between the two warring sides, he said. 

Humanitarian ceasefires between Ukrainian and Russian forces in Ukraine are not on the horizon right now, but maybe possible in a couple of weeks, Griffiths added. 

He noted that UN aid officials are planning to dispatch a humanitarian convoy in the next couple of days into the embattled eastern region of Donetsk and from there, send aid supplies into the Luhansk region.

Aid agencies have warned of the need to protect and evacuate civilians in the conflict. 

Griffiths had visited Moscow in early April before a trip to Kyiv, to try to obtain a humanitarian ceasefire and facilitate other aid interventions.

Last week, however, Guterres admitted that a halt to the fighting seemed unlikely and warned that it was too easy for the two sides to "blame each other when it goes wrong."

The official said he planned to visit Russia again after a trip to Turkey. Griffiths also said that keeping the port of Odesa open was of the "highest importance."

Odesa is home to full grain silos storing Ukraine's harvest, he said, calling it a "lifeline" to countries whose food supply has been impacted by the war.

On the diplomatic front, the Kremlin has accused Ukraine of constantly changing its stance when it comes to issues that have already been agreed at peace talks.

"Contacts continue at an expert level within the framework of the negotiation process", Kremlin spokesperson Dmitry Peskov told reporters on a conference call.

"Unfortunately the Ukrainian side is not consistent in terms of the points that have been agreed", he said.

"It is often changing its position and the trend of the negotiating process leaves much to be desired."

French President Emmanuel Macron said on Monday his dialogue with Putin had stalled.

Elsewhere, two British fighters captured in Ukraine by Russian forces have appeared on Russian state TV and asked to be exchanged for an ally of Russian President Vladimir Putin who is being held by the Ukrainian authorities.

The two captured Britons, Shaun Pinner, and Aiden Aslin spoke to the camera in footage that was broadcast on Russian state TV. 

The men both asked British Prime Minister Boris Johnson to exchange them for pro-Russian Ukrainian politician Viktor Medvedchuk.

A video appeal by Medvedchuk, in which he asks to be exchanged, was released on Monday by Ukraine's security service.

Medvedchuk is the leader of Ukraine's Opposition Platform, For Life party, and an ally of Putin, who has spent years advocating closer ties between Russia and Ukraine.

Both Pinner and Aslin had been fighting for the Ukrainian army in the southeastern Ukrainian city of Mariupol, which is now almost entirely under Russian control.

Russia sent its troops into Ukraine on February 24 in what it called "a special military operation." Since then, the West has imposed sweeping sanctions on Russia since then. 

Western capitals and Kyiv accuse Russia of unprovoked aggression. Russia denies targeting civilians in what it described as a “special operation” in the face of NATO’s eastward expansion towards Russian borders. 

Advocates have been calling on the West and the U.S. in particular to back the peace efforts to end the crisis. Critics accuse Washington of prolonging the war to benefit its own interests to contain Russian growing military and economic power. 

The conflict began after Moscow spent months requesting security guarantees from NATO concerning its eastward expansion. The guarantees were not provided to Moscow. 

However, the economic consequences of Western sanctions appear to have backfired on Western economies as well as having a dire effect on countries reliant on Ukrainian food supplies.