KYIV, Ukraine—Russian President Vladimir Putin told his German and French counterparts that Moscow was open to resuming negotiations with Ukraine, but that the West’s supply of weapons to the Kyiv government could exacerbate the crisis, the Kremlin said Saturday.

The comments came a day after U.S. officials signaled that the Biden administration is likely to approve the transfer of advanced, long-range rocket systems to Ukraine that the Kyiv government says are necessary to stop Russian advances in the Donbas region in the country’s east, where fighting has been intense.

Mr. Putin, in a phone call with French President Emmanuel Macron and German Chancellor Olaf Scholz, “confirmed the openness of the Russian side to the resumption of dialogue,” the Kremlin said.

But it said Mr. Putin cited the “dangerous nature of the ongoing pumping of Western weapons to Ukraine” and warned of “the risks of further destabilization of the situation and aggravation of the humanitarian crisis.”

Mr. Scholz and Mr. Macron initiated the conference call, which took place on Saturday and lasted around 80 minutes, the German chancellery said. The two European Union leaders asked Mr. Putin to start direct talks with Ukrainian President Volodymyr Zelensky and urged him to immediately end hostilities and withdraw Russian troops from Ukraine, the chancellery said.

The U.S. and allies have already supplied Ukraine with artillery, but it is insufficient on its own to counter Russian firepower, Ukrainian commanders say. The U.S. is now expected to provide multiple-launch rocket systems, or MLRS batteries, and a second type of mobile artillery launcher that can fire miles farther than any system now in Ukraine’s control.

News of the potential weapons transfer, and of Mr. Putin’s warning, came as Russia made fresh advances in the Donbas region.

Russian gains in the strategically important city of Severodonetsk may force Ukrainian troops to withdraw, a Ukrainian official said. The city is among the last major Ukrainian strongholds in Luhansk, which together with Donetsk makes up Ukraine’s eastern Donbas region. Moscow has focused its military campaign on the region after pulling its forces out of central Ukraine earlier in the war.

Ukrainian officials denied that Russians had encircled the Severodonetsk but said the Ukrainian forces might soon be forced to retreat while there was still a path out.

“The Russian army is destroying the city by shelling it with tanks, artillery, mortars,” Serhiy Haidai, the governor of Luhansk, said on Ukrainian television Saturday morning.

The previous night, he said Russian forces had taken the Myr resort complex on the northern edge of Severodonetsk, but he expected Ukrainians could defend the city for at least a few more days. “In order not to be surrounded, our troops may be ordered to retreat,” he said.

Also on Saturday, Russia shelled the southern port city of Mykolayiv, according to posts from Mayor Oleksandr Senkevych on social media. He said an unknown number of civilians had been killed in the attack.

Mr. Haidai described a dire situation for civilians in Severodonetsk. The road out of the city toward Ukrainian-held territory was no longer passable for trucks, he said, and cars were being shot at as they drove. Almost all residents were in basements or bomb shelters, he said, with no electricity, gas or running water. Most of the city’s 100,000 prewar residents have already fled.

Firefighters at a factory in Bakhmut, a city hit by shelling in eastern Ukraine.

Photo: afp contributor#afp/Agence France-Presse/Getty Images

Oleksandr Stryuk, the mayor of Severodonetsk, told Radio Liberty on Friday night that 90% of the residential buildings in the city had been damaged or destroyed, but many residents were afraid to leave because the only path out was so dangerous. Two-thirds of the city’s perimeter was controlled by Russian forces, he said.

“There is continuous shelling of residential neighborhoods,” he said. “There are casualties among the civilian population.”

Russia is taking steps to bolster troop levels for its military campaign, which Western military officials say has left its armed forces with heavy casualties.

Mr. Putin on Saturday signed a law annulling the upper age limit for military enlistment. Only Russians ages 18 to 40 and foreigners ages 18 to 30 previously were allowed to enlist. The new law would make it easier to recruit specialists, including those with medical and engineering expertise, Russian legislators have said.

The Kremlin said that Mr. Putin, in his call with the French and German leaders, also addressed the food shortages and price hikes resulting from Russia’s blockade of Black Sea ports. Mr. Putin said that Russia was ready to find options to ensure the unimpeded export of Ukrainian grain from the ports, the Kremlin said.

The Russian leader faulted the “erroneous economic and financial policies of Western countries, as well as the anti-Russian sanctions they imposed” as the main reason for disruptions to food supplies, the Kremlin readout said.

A couple reunites at a train station in Kharkiv after being separated by the war.

Photo: Bernat Armangue/Associated Press

An increase in the supply of Russian fertilizers and agricultural products would also help reduce strains on the global food market, but this would require “the lifting of the relevant sanctions restrictions,” the Kremlin said.

The Biden administration is expected to announce as early as next week that it will supply Ukraine with MLRS batteries, U.S. officials said Friday. Ukrainian officials say they urgently need the longer-range multiple-launch rocket systems to hold the ground in Donbas.

The administration approved $6 billion in direct military assistance to Ukraine as part of a $40 billion package earlier this month, but it has been reluctant to include the rocket systems in the next tranche of weapons supplies over concerns of provoking Russia.

It couldn’t be determined how many of the new systems the U.S. would provide, but one U.S. official said they could arrive in Ukraine within weeks. The U.S. also has said Ukrainian forces would need at least a week of training to use them.

Speaking to Naval Academy graduates on Friday in Annapolis, Md., President Biden said Mr. Putin is not only trying to take over Ukraine, “he’s literally trying to wipe out the culture and identity of the Ukrainian people, attacking schools, nurseries, hospitals, museums with no other purpose than to eliminate a culture.’’

—Illia Oliinyk and Bojan Pancevski contributed to this article.

Write to Ian Lovett at ian.lovett@wsj.com and Ann M. Simmons at ann.simmons@wsj.com