The two neighbours have been firing waves of explosive-packed drones at each other daily throughout the four-year war, as talks to end the conflict have gone nowhere.

Ukrainian President Volodymyr Zelensky on Sunday vowed to step up retaliatory strikes on Russian energy sites if Moscow did not halt its invasion.

"Russia can end its war at any moment. Prolonging the war will only expand the scale of our defensive operations," he said on social media.

The Ukrainian leader said his troops had struck a vessel equipped with cruise missiles at the port of Primorsk, in Russia's northwestern Leningrad region.

The region's oil export terminals have been hit several times in recent weeks, triggering massive fires that billow plumes of toxic black smoke into the atmosphere.

Kyiv says the strikes have knocked out billions of dollars' worth of Russia's vital export earnings.

Zelensky said Sunday three of Russia's so-called shadow tankers -- ageing vessels that ferry its sanctioned oil around the world -- were struck, one at Primorsk and two off the southern Black Sea port of Novorossiysk.

He posted night-vision footage of a naval drone approaching one tanker at Novorossiysk.

The Russian governor of the Leningrad region had earlier confirmed a fire at the port after Ukrainian attacks.

The extent of the damage was not immediately clear and Russian officials gave no details.

- 600 drones -

On the Russian side of the front line, two people were killed in the Belgorod border region, one near Moscow and a teenager in Russian-occupied southern Ukraine in separate attacks overnight and throughout Sunday.

In Ukraine, two were killed in the coastal Odesa region, one in the frontline Kherson region and another in an attack on the industrial city of Dnipro.

Photos from Dnipro showed the roof of a five-storey apartment block destroyed, wooden beams exposed and debris scattered into partially collapsed top-floor apartments.

Russia fired 268 drones and one ballistic missile in the overnight barrage, Kyiv's air force said.

Ukraine's army launched 334 drones at Russia, Moscow's defence ministry said.

Kyiv calls its attacks on Russia fair retaliation for Russia's nightly barrages of its cities.

Both sides deny targeting civilians.

Tens of thousands have been killed -- the vast majority in Ukraine -- since Russia invaded in February 2022.

In April, Russia fired a record number of long-range attack drones at Ukraine -- an average of more than 200 a day -- according to AFP analysis of data from Kyiv's air force.