With Ukraine set to mark both its independence from Soviet rule in 1991 and six months since Russian forces invaded, President Volodymyr Zelenskiy vowed that any Russian attacks in or around the date would provoke a powerful response.
Zelenskiy, who has led his country's resistance since Russian troops poured over the border on Feb. 24, also said Ukraine would restore its rule over the Crimea region - annexed by Russia in 2014 in a precursor to this year's invasion.
Despite his defiance, there was concern among Ukrainian and allied Western officials that Russia was preparing to attack the capital Kyiv once again.
The United States said it believed Russia would target civilian and government infrastructure in the next few days. US citizens should leave Ukraine "now" by their own means if it was safe to do so, the US Embassy said.
On the battlefields, Russian forces carried out artillery and rocket strikes in the Zaporizhzhia region in southeastern Ukraine, where fighting has taken place near Europe's largest nuclear power plant, Ukraine's military said. Russia and Ukraine have blamed each other for strikes on the plant.
Meanwhile leaders of dozens of countries and international organizations were taking part in the so-called Crimea Platform - most of them by video - in solidarity with Ukraine on the six-month anniversary of the invasion.
"To overcome terror, it is necessary to gain victory in the fight against Russian aggression," Zelenskiy, dressed in his customary military gear, told delegates in opening the forum.
"It is necessary to liberate Crimea. This will be the resuscitation of world law and order."
Zelenskiy told a news conference in Kyiv later that Ukraine would not agree to any proposal to freeze current front lines in order to "calm" Moscow, which now controls about 22% of Ukraine including Crimea.
He urged the world not to show fatigue with the war, saying this would pose a threat to everyone.
Italy's acting Prime Minister Mario Draghi told the summit Rome would continue to support Ukraine. "We are with you in your fight to resist Russia's invasion, restore Ukraine's territorial integrity, protect your democracy and independence," he said.
Zelenskiy also said Ukraine needed more weapons from Western supporters to help turn the tide against Russia.
Germany plans to deliver further arms, including air-defense systems, rocket launchers and precision munitions, to Ukraine worth over 500 million euros ($500 million) in 2023, a source told Reuters.
Zelenskiy had warned at the weekend that Moscow might try "something particularly ugly" in the run-up to Wednesday's Independence Day.
"They will receive a response, a powerful response," he said on Tuesday. "I want to say that each day ... this response will grow, it will get stronger and stronger."
Fears of intensified Russian attacks followed the killing of Darya Dugina, the daughter of a prominent Russian ultra-nationalist, in a car bombing near Moscow on Saturday. Moscow blamed the killing on Ukrainian agents, a charge Kyiv denies.
Kyiv has only rarely been hit by Russian missiles since Ukraine repelled a ground offensive to seize the capital in March.
The mood in Kyiv was calm on Tuesday, with many people still walking the streets, but signs of increased threat could be felt.
Authorities told Ukrainians to work from home where possible from Tuesday to Thursday, also urging people to take air raid warnings seriously and seek shelter when sirens sound.
The Kyiv city administration banned large public gatherings until Thursday, fearing that a crowd of celebrating residents could become a target for a Russian missile strike.
Stalemate
Russia sent its troops over the border in what it calls a "special military operation" saying it wanted to demilitarize its neighbor and protect Russian-speaking communities. Ukraine and its Western allies accuse Moscow of waging an unjustified, imperial-style war of aggression.
Six months on from the Russian invasion, which has caused thousands of deaths, forced over a third of Ukraine's 41 million people from their homes and destroyed whole cities, the conflict is largely locked in a stalemate.
In addition to Crimea, Russian forces control a large swathe of the south, including along the Black Sea and Sea of Azov coasts, and chunks of the eastern Donbas region. Peace prospects look almost non-existent.
"We feel good, trusting that victory will be on our side, only ours, there is no other option," Yevhen, a Ukrainian soldier, said as his frontline unit fired off several howitzer shells towards Russian positions from a field in the Donbas.
Russian shelling hit Kharkiv in the northeast - Ukraine's second largest city - around dawn on Tuesday, regional governor Oleh Synehubov said. A house was hit but no one hurt, he said.
In the south, Ukraine's southern military command said Russian forces pressed attacks along front lines of areas they occupy, including multiple rocket strikes on the town of Marhanets across the Dnipro River from the Russian-held Zaporizhzhia nuclear power plant. Two civilians were hurt and several homes, gas and water pipelines were damaged, it said.
It added that Ukrainian rockets and artillery had destroyed an ammunition depot and command post of a Russian airborne assault regiment in Chernobaevka in the Russian-occupied Kherson region, southwest of Zaporizhzhia.
Artillery and rocket fire near the nuclear reactor complex, has stirred international calls for the area to be demilitarized.
