Three were killed in Ukraine, one in Russia and one in Russian-occupied southern Ukraine, according to statements by local officials.

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

Ukrainian President Volodymyr Zelensky on Sunday vowed to step up Kyiv's 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, knocking out billions of dollars' worth of exports, according to Kyiv.

Zelensky said 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 black-and-white night-vision footage of a naval drone approaching one tanker at Novorossiysk.

Oil and gas revenues -- boosted by the war in the Middle East -- are critical for Russia's economy and funding its army.

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.

Russian attacks on Ukraine's southern Odesa region -- a key maritime logistics hub -- killed two people, and one person was killed in the frontline Kherson region.

- Moscow targeted -

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's attacks killed a 77-year-old man in the Moscow region, which surrounds the capital, and a 15-year-old boy in the Russian-occupied part of Ukraine's southern Zaporizhzhia region, the local governors 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.