Fighting continued at key points along the long front as Russian forces sought to expand their hold on territory in the Donetsk region of eastern Ukraine.
The Kremlin-appointed Donetsk leader, Denis Pushilin, claimed Russian forces were advancing near Vugledar, a strategically valuable town southwest of Donetsk city.
"Now we can say that units have established positions in the eastern part of Vugledar, and work is also being carried out in the vicinity," Pushilin said, according to Russian news agencies.
But Kiev rejected the claim, while conceding that the fighting there was tough.
"There are constant attempts to break through our defence," Ukraine's President Volodymyr Zelensky said late Sunday.
"The enemy... maintains a high intensity of attack."
Last week, Western countries including the United States and Germany approved the delivery of heavy battle tanks to Ukraine.
The move is a departure from a previous position that tanks were considered a step too far in their support for Ukraine and could risk a widening backlash from Russia.
But Biden on Monday appeared to draw the line on supplying American fighter jets to Ukraine.
"No," he said when asked by reporters at the White House if he was in favor of sending F-16s or others.
But European leaders said they were open to the idea, even if Ukraine has not yet formally requested advanced fighter aircraft from its allies for the war.
Analysts believe both Ukraine and Russia are gearing up for significant offensive movements in the coming months and Western aircraft could increase Kiev's strength, with its own air force significantly depleted by 11 months of war.
French President Emmanuel Macron said Monday he would not rule out giving fighter aircraft to Ukraine but warned against the risk of escalation in the conflict.
Macron had talks with Netherlands Prime Minister Mark Rutte, who has already floated the idea of sending Dutch F-16s to Ukraine.
"Nothing is excluded in principle," Macron said.
Any arms delivery "must not weaken the capacity of the French armed forces," he said, adding that France would have to be confident that the weapons would not be used to strike inside Russia, which could escalate the war.
"There is no taboo but it would be a big step," said Rutte.
Over in South America, President Luiz Inacio Lula da Silva said Brazil "has no interest in sending ammunition to Ukraine" during a meeting with German Chancellor Olaf Scholz -- the first Western leader to meet the leftist premier since he took office.
"Brazil is a country of peace... My suggestion is that we create a group of countries to sit at the table with Ukraine and Russia to try to reach peace," Lula said.
He added that he had discussed this idea with Macron and would bring it up with Biden and Chinese leader Xi Jinping in future planned visits.
Lula caused a stir last year when he claimed that Zelensky was "as responsible as" Russian President Vladimir Putin in the conflict.
Now, he said he believes Russia "made the classic mistake of invading another country's territory" -- but maintained his position of non-engagement.
"If one doesn't engage, then the two (sides) won't fight," Lula said.
With Ukraine's supplies of artillery munitions heavily depleted, France and Australia announced Monday a deal to jointly produce 155 mm shells for Kiev's forces.
"Several thousand 155 mm shells will be manufactured jointly," French Defense Minister Sebastien Lecornu said.
"There are some unique capabilities that exist in Australia and some synergies that can be achieved by Australia and France working together," said his Australian counterpart Richard Marles.
Meanwhile in Seoul, NATO Secretary General Jens Stoltenberg asked South Korea to "step up" military support for Ukraine, suggesting it reconsider its policy of not exporting weapons to countries in conflict.
There is an "urgent need for more ammunition", Stoltenberg said.
It is "extremely important that President Putin doesn't win this war," he stressed.
Tehran on Monday summoned a Ukrainian diplomat to protest at remarks made by an aide to Zelensky following unattributed weekend strikes that targeted an Iranian defense industry site in Isfahan that reportedly produced drones.
Although no link could be made between the weekend strikes and Ukraine, Mykhailo Podoliak tweeted about the "explosive night in Iran", highlighting the site's drone and missile production and oil refineries.
"Ukraine did warn you," he said.
Iran has been supplying attack drones to Russia for use against Ukraine.
Ukraine's charge d'affaires in Tehran was summoned to provide "an official and immediate explanation" for remarks it called "strange and biased", said Iran's foreign ministry.
