President Volodymyr Zelenskyy announced a deadly attack on a rail station Wednesday, the country's independence day, as he pledged Ukraine would fight "until the end" on the day that marked six months of war.
Washington separately warned Moscow was preparing to hold "sham" referendums in occupied areas that would seek to formalise its control.
The Russian missile strike on Chaplino station, in central Dnipropetrovsk region, had killed 15 people and wounded 50 others, Zelenskyy said at the start of a speech to the UN Security Council.
"Four passenger cars are on fire. Rescuers are working on the spot, but the death toll may rise unfortunately," he added. "This is our daily life."
Zelenskyy was speaking on the day the nation celebrated its 1991 independence from the Soviet Union -- and on the day marking six months since Russian President Vladimir Putin ordered troops to invade.
Over the weekend, he had warned that Russia might do something "particularly cruel" on Ukraine's independence day.
In Washington, a senior official warned that Russia could begin announcing referendums designed to formalise its control of occupied areas as soon as this week.
"Russian leadership has instructed officials to begin preparing to hold sham referenda," White House national security coordinator John Kirby said.
"In fact, we can see a Russian announcement of the first one or ones before the end of this week."
British Prime Minister Boris Johnson paid a surprise visit to Kyiv, hailing Ukraine's six-month long resistance, as sirens sounded throughout the day.
Putin had failed to account for the "strong will of Ukrainians to resist", Johnson said.
"You defend your right to live in peace, in freedom, and that's why Ukraine will win," he added.
Earlier the Ukrainian leader had issued his own defiant morning video address, declaring: "We don't care what army you have, we only care about our land. We will fight for it until the end."
Referring to Russia he vowed Ukraine "will not try to find an understanding with terrorists".
"For us Ukraine is the whole of Ukraine," he said. "All 25 regions, without any concession or compromise."
The US, meanwhile, announced $3 billion in fresh military aid.
The new funding will help Kyiv acquire more materiel for its armed forces, locked in a grinding war of attrition with Russian troops in the east and south, with neither side advancing significantly.
Johnson unveiled his own £54 million ($64 million) package of aid, including 2,000 "state-of-the-art drones" as well as anti-tank munitions.
Gatherings were banned in the capital Kyiv and Zelenskyy had urged citizens to be on guard against "Russian terror".
Nevertheless he and his wife marked a minute of silence for fallen Ukrainian soldiers and laid yellow and blue floral bouquets at a memorial in central Kyiv.
Johnson's visit was accompanied by other messages of support from Ukraine's allies.
United Nations Secretary-General Antonio Guterres called the anniversary of the start of Russia's war in Ukraine a "sad and tragic milestone".
European Commission President Ursula von der Leyen said the EU has been standing with Ukraine "from the very beginning" and "will be for as long as it takes".
Even Alexander Lukashenko, the authoritarian leader of Belarus congratulated Ukraine on its Independence Day, comments dismissed by a spokesman for the Ukrainian presidency.
Belarus offered its territory as a staging ground for Russia's invasion.
In the early days and weeks of Russia's invasion, Kyiv was under siege by Russian troops which reached the suburbs of the capital.
Moscow's offensive quickly faltered, and its forces withdrew in late March to regroup for assaults on Ukraine's east and south.
But in the capital, Ukrainians were sombre about the anniversary.
"Six months, the peace of life has been broken in every family," Nina, an 80-year-old pensioner, said on Independence Square on Tuesday.
"How much destruction, how many dead, how can we relate to it?" she asked.
The capital city's administration shut public service centres on Wednesday and Thursday, and shopping centres said they would close for the anniversary over safety concerns.
However in central Kyiv trailing crowds of people inspected dozens of disabled Russian tanks, trucks and armoured vehicles installed near the government quarter to showcase Ukraine's military prowess.
Candy floss vendors sold to curious visitors who peered down tank barrels and posed for selfies, draped in the Ukrainian flag.
Discussions continued on how to protect the Zaporizhzhia nuclear plant in southern Ukraine, occupied by Russian troops and threatened by shelling, which Moscow blames on Kyiv.
The two sides traded accusations at a Tuesday meeting of the UN Security Council on Zaporizhzhia.
Ukraine and its allies have demanded Russia pull its troops out of the plant -- Europe's largest nuclear facility -- and agree to a demilitarised zone.
And on Wednesday the head of Russia's state nuclear energy agency met the IAEA chief to follow up on the expected inspection.