Ukrainian President Volodymyr Zelenskyy has rejected Russia's claim it has conquered the key city of Bakhmut after months of fighting.
Mr Zelenskyy was speaking at the end of a surprise visit to the Group of Seven (G7) summit in Hiroshima.
Earlier on Sunday, Russia claimed to have won the bloody battle for the eastern Ukrainian city of Bakhmut.
The city is not strategically important for Kyiv or Moscow, but after becoming the subject of the longest battle of the war in Ukraine so far, it has taken on a symbolic importance for both sides.
While Mr Zelenskyy declined to share precise details, he said the city "is not occupied by Russia" currently.
During his visit to Japan, Mr Zelenskyy secured a new package of military aid from the United States.
The aid package included ammunition, artillery and armoured vehicles for Ukraine, which is reportedly planning a renewed counteroffensive in the coming months.
US President Joe Biden told the Ukrainian leader that the United States was doing all it could to strengthen Ukraine's defence in the war with Russia.
"We have Ukraine's back and we're not going anywhere," Mr Biden said.
Mr Zelenskyy thanked the US president for the support.
"We are very thankful. We will never forget. Thank you," he said.
Even before Mr Zelenskyy landed in Hiroshima on a borrowed French plane, G7 nations unveiled a slew of new sanctions and other measures meant to punish Moscow and hamper its war effort.
The United States agreed to allow training on potent American-made fighter jets, which could lay the groundwork for the eventual transfer of F-16s to Ukraine.
At a media conference before boarding a plane home, Mr Biden said the G7 "shared an unwavering commitment to stand with the brave people of Ukraine".
"Putin will not break our resolve," he said.
Russia says it's claimed key Ukrainian city
The assault on the largely destroyed city of Bakhmut was led by Wagner Group mercenaries.
Their leader Yevgeny Prigozhin claims to have finally pushed the Ukrainians out of the last built-up area inside the city.
Taking Bakhmut — which Russia refers to by its Soviet-era name of Artyomovsk — would represent Moscow's first big victory in the conflict in more than 10 months.
When asked to confirm whether Kyiv still controlled Bakhmut, Mr Zelenskyy responded: "It's a pity, it's a tragedy, but for today Bakhmut is only in our hearts."
The Ukrainian leader said that the fight had left nothing in Bakhmut but a lot of “dead Russians".
However, at a press conference, he later denied that these comments should be interpreted as a concession.
In Japan, Mr Zelenskyy compared the devastation of Bakhmut to Hiroshima after the atomic bomb was dropped in 1945.
"Although the enemy's using non-nuclear weapons, the ruins of our cities that have been burned to the ground through Russian bombs and artillery are similar to what I've seen here."
Mr Zelenskyy said Ukraine's military personnel in Bakhmut are "accomplishing a very important mission".
"There are no two or three interpretations of those words," he said.
A solemn visit to Hiroshima's peace park
Mr Zelenskyy also visited the Hiroshima Peace Memorial which serves as a reminder about the horrors that nuclear weapons can create.
He said the exhibition that shows blackened residue of a bomb blast victim, known as "the human shadow", was particularly powerful.
"There are lots of tragic pictures in the museum," he said.
"The children, little babies, before their death, and we have similar pictures, unfortunately. Only tears are in my eyes.
"I'm not a doctor to understand the sick attitudes Russians have towards us. The shadow which Putin wanted our country to look like. I found such an association."
Later, during a bilateral meeting, he thanked the Japanese prime minister Fumio Kishida for making his country's plight a focus of the G7.
Stephen Nagy, a professor of international relations at the International Christian University in Tokyo, said the visit allowed Japan to highlight the risks of war in the Indo-Pacific, while also helping Ukraine garner more sympathy to its cause.
"From Ukraine's standpoint, I think they want to create the narrative that what happened in Hiroshima could happen in Ukraine," he said.
"We saw Mr Putin vow to deploy tactical nuclear weapons in Belarus."
It is a warning that has caused grave distress to those who survived the atomic bombing in 1945.
Keiko Ogura, who was eight at the time, spoke with world leaders at the G7 about her experiences.
"I want Mr Putin to come and listen to my story," she said.
"Russia, stop the war. We, especially survivors, do not want more killing."
Zelenskyy courts developing nations
Mr Zelenskyy also met with leaders from countries in the so-called Global South, who were invited to take part in the summit despite not being members of the G7.
The Ukrainian leader met with Indian Prime Minister Narendra Modi — their first face-to-face talks since the invasion — and briefed him on Ukraine's peace plan.
India has avoided outright condemnation of Russia's invasion.
While India maintains close ties with the United States and its Western allies, it is also a major buyer of Russian arms and oil.
"The war in Ukraine is a big issue for the whole world," Mr Modi said to the Ukrainian president.
"It has also had many effects on the whole world. But I don’t consider it to be just an issue of economy or politics. For me, it is an issue of humanity."
In the final communique, G7 leaders reaffirmed their commitment to Ukraine, and vowed to intensify the pressure on Russia.
"Russia's brutal war of aggression represents a threat to the whole world in breach of fundamental norms, rules and principles of the international community," the G7 said.
"We reaffirm our unwavering support for Ukraine for as long as it takes to bring a comprehensive, just and lasting peace."
