KYIV — Thirteen people have been killed and others wounded in a series of explosions in the separatist-run city of Donetsk in eastern Ukraine, according to its Russian-backed mayor.

Alexei Kulemzin blamed "punitive" Ukrainian shellfire for the deaths. There is no comment from Ukrainian officials.

Donetsk has been controlled by Russia's proxy authorities since 2014.

They have repeatedly accused Ukrainian forces of targeting the city.

Independent confirmation is hard to come by on the ground in separatist-held areas of the east. However, local authorities said nine 150mm shells were fired at the Kuibyshevsky district of Donetsk, from a village to the west of the city.

Local leader Denis Pushilin accused Ukraine of deliberately targeting civilians at a bus-stop, a shop and a bank.

Although Russian forces have seized areas of the Donetsk region further south since the invasion began in February, they have struggled to push the Ukrainian army back from the outskirts of the city itself.

Ukrainian forces have launched counter-offensives in the south as well as the north-east, and their most dramatic progress has come this month in the northern Kharkiv region.

The head of Luhansk region, Serhiy Haidai, shared video of a Ukrainian tank crossing a pontoon bridge, and said Ukraine now controlled the left bank of the Oskil river, seen as the front line in north-east Ukraine.

If Ukrainian forces are able to maintain a foothold on the eastern side of the Oskil, it will represent a breakthrough. Haidai said the next target would be liberating the city of Lyman, which was seized by Russian forces in May.

"Luhansk region is right next door. De-occupation is not far away," he claimed.

In his nightly address late on Sunday, President Volodymyr Zelensky said that recent days may have looked like a lull of sorts: "But there will be no lull. There is preparation for the next series... For Ukraine must be free. All of it."

Russia was accused on Monday of targeting a nuclear plant in the south.

Ukrainian nuclear operator Enerhoatom said a rocket landed 300m (1,000ft) from nuclear reactors at the country's second-largest plant in Mykolaiv region, called the South Ukraine plant.

The UN's International Atomic Energy Agency (IAEA) said it was told by Enerhoatom that the impact temporarily disrupted three power lines, but the plant remained connected to the grid, with all three reactors functioning normally.

Ukraine's - and Europe's - biggest nuclear plant at Zaporizhzhia has come under repeated fire since it was occupied by Russian forces at the start of the war and the IAEA has called for a safety zone to protect it. The IAEA said the situation there "remains fragile and precarious". Its six reactors are in a shutdown state, but are getting essential power to remain safe.

Meanwhile, the Kremlin has denied allegations of war crimes after some 450 bodies were uncovered in burial sites in the liberated city of Izyum.

The discovery has prompted the European Union's presidency, currently held by the Czech Republic, to call for an international tribunal into Russia's invasion.

Kremlin spokesman Dmitry Peskov said the revelations were "a lie, and of course we will defend the truth in this story". He said the "scenario" was the same as in the town of Bucha near Kyiv, where local authorities say at least 458 civilians were murdered during Russia's occupation.

President Vladimir Putin said on Friday that Ukrainian counter-offensives would not change Russia's military plans in the east of Ukraine.

The ministry of defense in Moscow posted video on Monday that it said showed attack helicopters destroying Ukrainian manpower and equipment.

However, according to the UK's defense intelligence update, it was "highly likely" that Russia had lost four combat jets in Ukraine in the past 10 days as its air force took greater risks to support ground forces under pressure from the Ukrainian advances.

The Institute for the Study of War says Russia is relying increasingly on "irregular volunteer and proxy forces" instead of its conventional military units.

Russia says it is fighting neo-Nazis in Ukraine - a claim widely dismissed - and that it is threatened by the Nato alliance's strong relations with Ukraine.

Since the invasion on 24 February, the UN has recorded at least 5,718 civilian deaths, with 8,199 injured, and more than seven million Ukrainians have been recorded as refugees across Europe.

The actual civilian death toll is believed to be thousands higher. Tens of thousands of combatants have been killed or injured.

Russia, a global energy supplier, is locked in an economic struggle with the West which imposed sweeping sanctions in response to the invasion. — BBC