The administration of United States President Joe Biden says it is preparing a large aid package for Ukraine in the coming weeks as a debate continues over whether Ukrainian forces should be allowed to use Western-supplied weapons to strike farther inside Russia.

White House National Security Adviser Jake Sullivan said on Saturday that Washington is working on a “substantial” round of further assistance for Kyiv and Biden will meet his Ukrainian counterpart, Volodymyr Zelenskyy, this month.

“I do think we need a comprehensive strategy for success in this war, and that is what President Zelenskyy says he is bringing,” Sullivan said in remarks delivered via videolink at the Yalta European Strategy conference in Ukraine’s capital.

“So we are very much looking forward to sitting down and talking that through, and President Biden is eager for that conversation,” he said.

Sullivan made the remarks as Ukraine has been pushing for more long-range weaponry to hit targets deeper inside Russian territory, a development that analysts worried could escalate tensions between Moscow and Kyiv’s Western backers.

More than two and a half years since Russia launched a full-scale invasion of its neighbour in February 2022, Ukrainian forces have struggled in recent weeks as Russia has made progress around Pokrovsk, a key transit hub in eastern Ukraine.

Pokrovsk’s capture could enable Moscow to open new lines of attack and complicate Ukrainian logistics.

Sullivan said recent developments in the area are of “unique concern”.

The White House adviser added that Biden, who is not seeking re-election and whose term ends in January, is working to put Ukraine in the “best possible position to prevail” during his final months in office.

“President Zelenskyy has said that ultimately this war has to end through negotiations, and we need them to be strong in those negotiations,” Sullivan said.

Meanwhile, NATO Military Committee Chairman Rob Bauer said he supported Ukraine’s use of long-range weapons to strike inside Russia.

“Every nation that is attacked has the right to defend itself. And that right doesn’t stop at the border of your own nation,” the admiral said on Saturday at a committee meeting.

Russian President Vladimir Putin has warned that if Western nations allow Ukraine to use such weapons to strike inside Russia then it will mean NATO would be “at war” with his country.

“This would in a significant way change the very nature of the conflict. It would mean that NATO countries, the US, European countries, are at war with Russia,” Putin told Russian state TV on Thursday.

“And if this is so, then, bearing in mind the change in the very essence of this conflict, we will make appropriate decisions based on the threats that will be created for us.”

A day later, Russia’s ambassador to the United Nations delivered a similar message to the UN Security Council in New York.

“The facts are that NATO will be a direct party to hostilities against a nuclear power. I think you shouldn’t forget about this and think about the consequences,” Vassily Nebenzia told the 15-member council.