Ukrainian President Volodymyr Zelensky has said Russia's conditions for a ceasefire on the Black Sea were "unrealistic," and that Russia was trying to drag out talks in order to grab more land.
Mr Zelensky also said that lifting sanctions on Russia at present would be a "disaster for diplomacy."
He was speaking after meeting dozens of European leaders in Paris for a summit aimed at providing guarantees for Ukraine in the event of a ceasefire.
Ukraine's army has rejected a Russian claim that it had targeted energy sites, after Moscow accused Kyiv of breaching an agreement reached with the United States on Tuesday to halt such aerial attacks.
Ukraine's General Staff wrote on social media that a "statement by the Russian defence ministry" on alleged violation of the cessation of strikes against energy facilities "is not true".
Ukraine's foreign miinstry said neither country had hit each other's energy infrastructure since 25 March.
The United States announced separate agreements with Ukraine and Russia on Tuesday to pause their strikes in the Black Sea and against each other's energy targets.
Meanwhile, Russian forces are stepping up attacks along Ukraine's northeastern border in a bid to break into the Sumy region, Kyiv's top general has said.
Sumy borders Russia's Kursk region, where Ukrainian forces have been all but pushed out of Russian territory they captured in a cross-border offensive last August.
Oleksandr Syrskyi, while visiting units in the Sumy region, said his troops were conducting defensive and assault operations to retain a buffer zone inside Kursk.
"Our soldiers continue to conduct a defensive operation to prevent the enemy from breaking deep into Ukrainian territory," he added on Facebook.
Mr Syrskyi's comments followed a fresh warning by Mr Zelensky, published in Le Figaro, that Russia was preparing a spring offensive against the Sumy and Kharkiv regions.
A spokesman for Ukraine's border guard told Ukrainian media yesterday that Russian sabotage groups had stepped up cross-border raids into the Sumy region.
Separately Russian shelling has killed at least two people, damaged railway infrastructure and partially disrupted power supplies in Ukraine's southern city of Kherson, officials said.
The regional governor said on Telegram that "massive shelling" killed a 55-year-old woman and a man at a public transport station.
At least five people were wounded, Roman Mrochko, the head of Kherson's military administration, said on Telegram.
A city train station came under shelling as well, Ukraine's state railway company said. Power and water supply lines were damaged, as well as a train carriage, it added.
As a result of the partial power cuts, trolley-bus services were temporarily suspended, Mr Mrochko said.
Kherson regularly comes under Russian shelling and drone attacks.
Moscow's troops occupy part of the surrounding region, three years on from Russia's February 2022 full-scale invasion.
Zelensky urges US to remain 'strong' amid Russian demands
Mr Zelensky earlier said that he wanted the United States and Ukraine's other Western allies to remain strong in countering Russia's demands and narrative in its three-year war on his country.
But Mr Zelensky said that pro-Kremlin comments by the United States were weakening US pressure on Russia and bringing peace no closer.
Mr Zelensky, interviewed by a panel of European journalists in Paris on the eve of this afternoon's meeting of European leaders, said he had agreed to proceed with ceasefire talks to ensure a resumption of US aid and intelligence sharing.
But Russia has placed additional demands on implementation of a ceasefire deal in the Black Sea and on energy targets, focusing mainly on easing sanctions pressure on Russia, he said.
"They are posing conditions on sanctions to the American side. If America stands strong ... we are standing on our own land and are defending ourselves," Mr Zelensky said.
"We have shown our resilience. And now it is very important for our partners to be at least as resilient as we are, though they have more capabilities," he added.
Asked whether he believed the Americans would remain strong, he answered: "I hope so. God bless they will."
Mr Zelensky expressed gratitude for US assistance in the war, but said the United States had become "influenced" by Russian narratives of the conflict.
"We cannot agree with these narratives. We are fighting for ourselves and we will fight against these narratives wherever they appear because there is simply no other way," he said.
"The only thing we can do is reveal more truth. We must demonstrate with our own examples that we are fighting on the battlefield," he added.
He said the US envoy to the Middle East, Steve Witkoff, "often repeats Kremlin narratives", referring to Mr Witkoff's favourable statements about President Vladimir Putin's intentions and justification for Russian policies.
The US has softened its rhetoric towards Russia in recent days, with Mr Witkoff saying he did not "regard Putin as a bad guy".
"I don't think this brings us any closer to peace. Unfortunately, I believe it only weakens the pressure from the Americas on Russia," Mr Zelensky said.
"I have spoken with President [Donald] Trump many times and we are working at the intelligence level to exchange real, truthful information. Because Witkoff's statements are a major obstacle for us," he added.
Russia invaded Ukraine in February 2022 and now controls about 113,000sq.km, or about 20%, of Ukraine, with fighting along a 1,000km frontline.
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