Russia pressed its onslaught on eastern Ukraine Saturday, saying it had captured the strategic town of Lyman and claiming to have surrounded the urban center of Severodonetsk.
"The situation is very difficult, especially in those areas in Donbas and Kharkiv region, where the Russian army is trying to squeeze at least some result for itself," Ukrainian President Volodymyr Zelenskyy said in his evening address.
His remarks came as Russia, in another exercise in military muscle-flexing, said it had successfully tested hypersonic missiles in the Arctic.
A Ukrainian official denied Moscow's claim that Severodonetsk had been encircled, saying government troops had repelled Russian forces from the outskirts of the key city.
After setbacks in its attacks on Kyiv and Kharkiv, Russia has been waging all-out war for the eastern Donbas -- Ukraine's industrial heartland, where Zelenskyy has accused Moscow of carrying out a "genocide" and where another Ukrainian official denounced Russia's "scorched-earth tactics".
"The town of Krasny Liman has been entirely liberated from Ukrainian nationalists," the Russian defence ministry said Saturday, using Moscow's name for Lyman and confirming an announcement made a day earlier by pro-Moscow separatists.
Lyman lies on the road to Severodonetsk and Kramatorsk.
Russian forces have been closing in on Severodonetsk and nearby Lysychansk in Lugansk province.
Regional governor Sergiy Gaiday said Russian shelling continued on Severodonetsk as Ukrainian soldiers fought to oust the invading forces from a hotel on its edges.
A Lugansk police official cited by Russia's state news agency RIA Novosti said late Friday that Severodonetsk was "now surrounded".
But Gaiday told Ukrainian television that "Severodonetsk has not been cut off... there is still the possibility to deliver humanitarian aid."
Regional officials on Saturday reported the deaths of three civilians at Russian hands, one each in the eastern towns of Bakhmut and Avdiivka, and one in the southern city of Mykolaiv.
"Ukraine will definitely win this war," Zelenskyy told Dutch television station NOS.
"The question is at what cost."
The Russian assault on Ukraine, launched on February 24, has left thousands dead on both sides and forced 6.6 million people out of the country.
Moscow has gained control over parts of eastern and southern Ukraine, including port cities Kherson and Mariupol.
Other Ukrainian ports have been cut off from the world by Russian warships, blocking grain supplies from being transported out.
Russia and Ukraine supply about 30 percent of the wheat traded on global markets.
Russia has tightened its own exports and Ukraine has vast amounts stuck in storage, driving up prices and cutting availability across the globe.
President Vladimir Putin has repeatedly rejected any responsibility, instead blaming Western sanctions.
But on Saturday, he told French President Emmanuel Macron and German Chancellor Olaf Scholz in a phone call that Russia was "ready" to look for ways to allow more wheat onto the global market.
"Russia is ready to help find options for the unhindered export of grain, including the export of Ukrainian grain from the Black Sea ports," the Kremlin quoted him as saying.
He also called for the lifting of sanctions to allow "an increase in the supply of Russian fertilisers and agricultural products" onto the global market.
Macron and Scholz urged Putin to hold "direct serious negotiations" with Zelenskyy, the German chancellor's office said.
And they demanded Russia free 2,500 Ukrainian fighters taken prisoner after surrendering earlier this month at a sprawling steelworks in Mariupol.
Zelenskyy said his country was doing everything possible to defend the Donbas from intense artillery and missile strikes.
"Every day we work to strengthen our defence," he said Saturday. "Every day we are approaching the situation when our army will prevail over the invaders technologically and in striking force."
To further help the Ukrainians, Washington is preparing to send advanced long-range rocket systems, US media reports said.
Pentagon spokesman John Kirby did not confirm the plans to deliver the M270 Multiple Launch Rocket System, highly mobile equipment capable of firing up to 300 kilometres (186 miles) that Kyiv has said it badly needs.
But he said Washington was "still committed to helping them succeed on the battlefield".
In a phone call Saturday, British Prime Minister Boris Johnson told Zelenskyy his country would continue to help "provide the equipment they need", his office said.
But Putin warned Macron and Scholz that ramping up arms supplies to Ukraine would be "dangerous" and risk "further destabilisation".
He spoke after his army said it had successfully fired one of its Zircon hypersonic cruise missiles some 1,000 kilometres (625 miles) across the Arctic.
Ukrainian Defence Minister Oleksiy Reznikov meanwhile said on Facebook that his troops had gained valuable experience in operating foreign-made heavy artillery that would have been "impossible to imagine" as recently as March.
In the past month and a half, he said, "we have received more NATO-standard artillery shells than there are Soviet shells available!"
As Zelenskyy seeks to ramp up international pressure on Moscow, he will speak to EU leaders at an emergency summit Monday on an embargo on Russian oil. Agreement on the measure is being held up by Hungary, whose Prime Minister Viktor Orban has close relations with Putin.
But Moscow said Russia expects to receive one trillion rubles ($15 billion) in additional oil and gas revenues this year, a windfall from the sharp rise in oil prices caused in part by its invasion of Ukraine.