Ukraine faced new turmoil Sunday with President Volodymyr Zelensky announcing he sacked his top two law enforcement officials, in the government's most serious shakeup since Russia launched its deadly invasion in February.

The domestic crisis came as the European Union prepared to discuss tightening sanctions against Russia on Monday, and as Kyiv accused Moscow of launching fresh strikes on multiple residential areas in eastern and southern Ukraine.

The new attacks came after Moscow announced it would step up its military operations and Ukraine accused Russia of installing missile launchers at Europe's largest nuclear plant.

Zelensky said he was firing prosecutor general Iryna Venediktova and security chief Ivan Bakanov amid a high number of cases of suspected treason by Ukrainian law enforcement officials.

In a national address, Zelensky said over 650 cases of suspected treason and aiding and abetting Russia by Ukrainian security officials are currently being investigated, including 60 cases of officials who he said have remained in territories occupied by Russia and are "working against our state."

"Such a great number of crimes against the foundations of national security and the connections established between Ukrainian law enforcement officials and Russian special services pose very serious questions to the relevant leaders," Zelensky said.

"Each such question will be answered."

Zelensky also highlighted the devastating military might Moscow has used against Ukraine, saying that as of Sunday Russian forces have launched more than 3,000 cruise missiles on targets in Ukraine.

'High price'

With the conflict grinding on and increasingly spilling out into global energy and food crises, EU foreign ministers are considering banning gold purchases from Russia.

And more Russian figures could also be placed on the EU blacklist.

"Moscow must continue to pay a high price for its aggression," European Commission President Ursula von der Leyen said after forwarding the proposed measures.

Brussels is expected to hold initial sanctions discussions Monday, but not make a same-day decision, according to a senior EU official.

The heaviest fighting continues to focus on the industrial east of Ukraine, and on Sunday, Donetsk regional governor Pavlo Kyrylenko accused Moscow of shelling "civilian infrastructure, especially education institutions".

But Igor Besukh, a chef in the local city of Kramatorsk, just 20 kilometres (12 miles) from the frontline, was determined to stay strong in the face of Russian attacks.

Even after a Friday missile strike on Kramatorsk, which he admits scared him, Besukh continued operating his sushi restaurant, one of the few places still open in the city.

"War is war, but lunch must be served on time," he said, quoting a popular saying.

'Massive shelling'

Near Ukraine's Black Sea coast, the southern city of Mykolaiv came under "massive shelling" Sunday, regional governor Vitaliy Kim said.

Kim added that several residential areas were shelled in the region a day earlier, with three people killed in the village of Shevchenkove and one woman killed in Shyrokiv where a "residential building was destroyed".

In a BBC television interview broadcast Sunday, the head of Britain's armed forces, Admiral Tony Radakin, estimated that 50,000 Russian soldiers had been killed or wounded in the invasion with nearly 1,700 Russian tanks and some 4,000 armoured fighting vehicles destroyed.

Radakin suggested that Russia's land forces may pose less of a threat now, but more than 20 weeks since the invasion began, Moscow said Saturday it would step up its military operations.

Defence Minister Sergei Shoigu "gave the necessary instructions to further increase" military pressure, according to his ministry.

The orders come after Ukraine's atomic energy agency accused Russians of installing missile launchers at the Zaporizhzhia nuclear plant and using the facility to shell the Dnipro region.

Russia's defence ministry said in its daily briefing Sunday that it destroyed a "warehouse for Harpoon anti-ship missiles delivered to Ukraine by NATO" in the port city of Odessa.

Ukraine denied the claim, saying Russia destroyed the "storage facility" of a company with no military links.

Hundreds of kilometres from the frontline, Ukraine said missile strikes earlier in the week left 24 dead in the central city Vinnytsia, triggering international condemnation.

Russia's defence ministry said it had targeted a meeting in Vinnytsia of the "command of the Ukrainian Air Force with representatives of foreign arms suppliers".

But a senior US defence official said on condition of anonymity that he had "no indication" there was a military target nearby.

Meanwhile, in Russia, police on Sunday detained journalist Marina Ovsyannikova, who in March interrupted a live TV broadcast to denounce the military action in Ukraine, her lawyer said.

No official statement has been made, but her detention comes days after Ovsyannikova, 44, demonstrated alone near the Kremlin holding a placard criticising Russia's invasion and President Vladimir Putin.