"This is terror. They are going to bomb our Ukrainian cities even more, they are going to kill our children even more subtly. This is the evil that has come to our land and must be destroyed," Zelenskiy said on a short video message.

"Russia's criminal actions against Ukraine bear signs of genocide," he said, quoted by Reuters.

Zelensky also said Ukraine has lodged a complaint against Russia at the International Court of Justice in The Hague to get it to halt its invasion.

"Russia must be held accountable for manipulating the notion of genocide to justify aggression," Zelensky declared in a tweet, as reported by AFP.

"We request an urgent decision ordering Russia to cease military activity now and expect trials to start next week.

The ICJ, which is based in the Netherlands capital The Hague, does not have a mandate to bring criminal charges against individual Russian leaders behind the four-day-old invasion.

But it is the world's top court for resolving legal complaints between states over alleged breaches of international law. It is the supreme judicial institution of the United Nations.

The Kremlin has tried to justify its operation to "demilitarise" Ukraine as an attempt to prevent the alleged persecution of the country's Russian-speaking minority.

But the international community has roundly condemned the invasion as a flagrant breach of international law, and many Ukrainian civilians have volunteered to defend their country.   

Meanwhile, Turkey called Russia's invasion of Ukraine a "war" on Sunday in a rhetorical shift that could pave the way for the NATO member nation to enact an international pact limiting Russian naval passage to the Black Sea.

Under the 1936 Montreux Convention, Turkey has control over the Dardanelles and Bosphorus straits that connect the Mediterranean and Black seas and can limit the passage of warships during wartime or if threatened.

Balancing its Western commitments and close ties to Moscow, Ankara has said the Russian attack is unacceptable but until Sunday had not described the situation as a war.

"On the fourth day of the Ukraine war, we repeat President (Tayyip) Erdogan's call for an immediate halt of Russian attacks and the start of ceasefire negotiations," presidential spokesperson Ibrahim Kalin said on Twitter.

Fahrettin Altun, Turkey's communications director, said "we are witnessing yet another war in our region", and repeated Erdogan's offer to mediate

Kyiv has appealed to Ankara to block any more Russian warships from passing toward the Black Sea, from which Moscow has launched one of its incursions on Ukraine's southern coast.

But Turkey's foreign minister said on Friday that Russia had the right under Montreux to return ships to their home base, which could limit any Turkish policy shift.