Ukrainians revelled in a surreal display of burnt-out Russian tanks and armour laid out this week as war trophies in central Kyiv to mark the 31st anniversary of independence, but fears of fresh Russian attacks lurked behind their show of defiance.

An air raid siren perforated an eerie calm in Kyiv on the morning of Wednesday's Independence Day following dire warnings that Russia could launch fresh attacks on major cities. Kyiv has warned Moscow of a powerful response if that happens.

The anniversary fell exactly six months after Russia sent tens of thousands of troops into Ukraine.

In an emotional speech to his compatriots, Zelensky said the attack had revived the nation's spirit.

"A new nation appeared in the world on Feb. 24 at 4 in the morning. It was not born, but reborn. A nation that did not cry, scream or take fright. One that did not flee. Did not give up. And did not forget," he said.

The 44-year-old leader, speaking in front of Kyiv's central monument to independence in his trademark combat fatigues, vowed to recapture occupied areas of eastern Ukraine as well as the Crimean peninsula, which Russia annexed in 2014.

"We will not sit down at the negotiating table out of fear, with a gun pointed at our heads. For us, the most terrible iron is not missiles, aircraft and tanks, but shackles. Not trenches, but fetters," he said.

He and his wife later attended a service in Kyiv's St. Sophia cathedral along with religious leaders from all of Ukraine's major faiths.

Russia has made few advances in Ukraine in recent months, after its troops were pushed back from Kyiv in the early weeks of the war. Ukrainian soldiers on the front line in the east said they were more motivated than their enemy.

"All of our people are cheering for us. The whole country is, and other countries who help us too. Our fighting spirit is greater than theirs," a soldier called Yevhen told Reuters, declining to give his last name.

Russian Defence Minister Sergei Shoigu told a meeting of defence ministers in Uzbekistan that Russia had deliberately slowed down what it refers to as its "special military operation" in Ukraine to avoid civilian casualties.