Ukrainian troops have gone over to the offensive in the embattled eastern city of Severodonetsk, according to military analysts, while Moscow confirmed attacks on Kiev.
"Over the last 24 hours, Ukrainian forces have counterattacked in the contested city of Severodonetsk in eastern Ukraine, likely blunting the operational momentum Russian forces previously gained through concentrating combat units and firepower," Britain's Defence Ministry said based on information from the British intelligence services.
The troops committed by Russia in the region include poorly trained and equipped recruits from the Russian-led separatist forces of the self-declared Luhansk People's Republic. They lack the heavy equipment used by Russian units, the ministry tweeted.
Russia also again fired missiles at Kiev and a suburb of the Ukrainian capital on Sunday morning, according to Moscow and Ukraine's general staff.
Military and civilian infrastructure were hit, Kiev's military command announced in what was the worst attack the region has seen in several weeks.
One Russian missile also flew dangerously close to the South Ukraine Nuclear Power Plant, 350 kilometres south of Kiev, according to the Ukrainian nuclear power operator Energoatom.
The Russian aggressors "still do not understand that even the smallest fragment of a missile hitting a working power unit can cause a nuclear catastrophe and radiation leak," the operator wrote on Telegram, accusing Russia of "nuclear terrorism."
Kiev Mayor Vitali Klitschko also reported missile strikes on his Telegram channel, saying the south-eastern district of Darnytskyi and Dniprovskyi in the west of the metropolis were affected and that a man had to be treated in hospital after the attack.
The suburb of Brovary, which has been shelled repeatedly during the war, was also hit by rockets, authorities reported.
Ukrainian presidential adviser Mykhailo Podoliak called the attack an "act of terrorism" and demanded fresh sanctions on Russia, as well as for Ukraine to be supplied with heavy weapons. "Today's missile attacks on Kiev have only one aim; to kill as many Ukrainians as possible," he tweeted.
The extent of the damage was initially unclear but photos and videos of fire and clouds of smoke were shared on social media and in many, the sound of impacts could be heard.
Later, the Russian military confirmed the Kiev attacks and claimed T-72 tanks supplied to Ukraine by Eastern European allies had been destroyed, along with other military hardware stored at a repair yard for railway carriages.
Russian Defence Ministry spokesperson Igor Konashenkov also said there had been several missile attacks in Donetsk including on Kramatorsk, destroying a workshop for repairing weapons, ammunition dumps and command points, and killing more than 350 Ukrainian soldiers.
Konashenkov further claimed that Russian aircraft had downed a Ukrainian MiG-29 jet near Slovyansk in Donetsk and that a Ukrainian Antonov An-26 transport plane was shot down by Russian anti-aircraft fire near Odessa.
The latest attacks came as a report said that Madrid would provide modern tanks to Ukraine.
Spain plans to supply Ukraine with modern main battle tanks made in Germany, providing the country with state-of-the-art Western tanks for the first time, according to national daily El Pais.
The Leopard 2 A4 tanks are currently in storage and will have to be readied for use before deployment, the newspaper said, citing the Defence Ministry. Ukrainian tank crews are to be trained initially in Latvia and later in Spain, according to the information.
Madrid is also preparing to supply a battery of Italian-manufactured Aspide anti-aircraft missiles, the report said. The system is no longer in service with the Spanish military.
Later, Russian President Vladimir Putin vowed a "strong response" if Ukraine's allies supply the country with Western-made long-range missiles.
"If they supply, then we will draw the corresponding conclusions from this and deploy our means of destruction, of which we have enough, to deliver blows to those objects that we are not yet attacking," he told state broadcaster Rossija 1.
The West's aim in supplying arms to Ukraine was to draw out the conflict, Putin said.
The Russian leader appeared relaxed about the delivery of modern HIMARS multiple rocket launchers announced by the United States, saying it would not change the balance of forces fundamentally.
"There's nothing new here," he said, noting that the Ukrainian army already had similar, Russian-made systems and that US deliveries would just replace systems that had been destroyed.
Precisely which missiles were deployed would be decisive, Putin said. According to his information, missiles with ranges of between 45 and 70 kilometres would be supplied, similar to Russia's Uragan, Smerch and Grad systems. Ukraine had 515 such systems when the "operation" began and is still thought to have 360, he said.
The situation was similar with the artillery the West has supplied to Ukraine, he said. "By all appearance, this is about replacing what has been lost, been destroyed in action," Putin said. He added that Russian anti-aircraft forces had destroyed most of Ukraine's military drones. "Our anti-aircraft systems are cracking them like nuts."
Meanwhile Pope Francis appealed for an end to the violence.
"As the fury of destruction and death runs rampant and antagonisms flare up, feeding an ever more dangerous escalation for all, I renew my appeal to the leaders of nations: Please, do not plunge humanity into disaster," he told crowds on Sunday.
The pope called for concrete negotiations to begin for a ceasefire and a lasting solution. "Let the desperate cry of the people who are suffering be heard," he said, demanding a halt to the destruction of towns and villages.
