Ramstein Air Base, Germany — Ukrainian President Volodymyr Zelenskyy rarely attends the security summits organized by his Western partners in person, but on Friday, he paid his first visit to the sprawling Ramstein Air Base in Germany, where U.S. Secretary of Defense Lloyd Austin declared in his opening remarks of the latest such meeting that “it is a critical moment” in the war ignited by Russia’s 2022 full-scale invasion.

The gathering of the Ukraine Defense Contact Group comes days after Zelenskyy’s nation suffered its single worst attack of the year, when two ballistic missiles slammed into a military academy in Poltava, killing dozens of people and wounding more than 170 others.

The meeting in Germany also comes on the heels of a wider, blistering Russian barrage of missile and drone attacks across Ukraine. It comes after Ukraine announced the fatal crash of an F-16 fighter jet during that barrage. And it comes one month after Ukrainian forces invaded Russia’s western Kursk region, seizing hundreds of square miles of territory.

“The Kremlin’s army of aggression is now on the defensive on its own turf,” Austin said Friday of the Kursk offensive.

“This is the land from which Putin was preparing to expand the war into Ukrainian territory. He was preparing to launch a new offense against our city of Sumy,” said Zelenskyy.

The Ukrainian president has said controlling that Russian ground has created a buffer zone between President Vladimir Putin’s forces and Ukrainian territory. But if he was hoping the incursion would force Putin to divert his troops from the battlefields of eastern Ukraine, that hasn’t happened.

“This is a clear choice by Moscow,” Zelenskyy said. “Putin wants more Ukraine to occupy than human security. He doesn’t care about Russian land and people. He just wants to grab as much of our land and as many of our cities as possible.”

On the overnight flight from Andrews Airforce Base near Washington to Ramstein, Chairman of the Joint Chiefs of Staff General Charles “CQ” Brown spoke of the Kursk incursion as just one part of Ukraine’s war strategy.

“What they’ve been able to do in Kursk is to be able to stay successful there and at the same time be able to protect their key lines of communication at Pokrovsk,” he said. “So, it’s a combination of us working with them to identify the capabilities that they have, how they use the forces that they do have, to be successful in both of those areas.”

Ukraine has been losing ground around Pokrovsk, however, and the fall of that logistics hub to Russia would threaten critical supply lines for Ukrainian forces already struggling to hold parts of the roughly 600-mile front line in eastern Ukraine.

It’s a vulnerability that U.S. forces are keeping a close eye on.

“The Russians have been pushing toward their defensive lines and [they’re] starting to gain territory, so what we’re really paying attention to is the rate of advance by the Russians, and when they can get into range of artillery,” said Brown. “This is something we’ll continue to watch on and continue to work with Ukrainians, [on] how to defend that territory.”

In his own opening remarks at Ramstein, Zelenskyy struck a familiar refrain, stressing that his country needs more air defense systems and more long-range weapons, and soon.

“Thank you so much again,” he told the gathered Western military commanders, “but the number of air defense systems have not yet been delivered easily. This is what was agreed upon, and this is what has not been fully delivered. The world has now systems to ensure that Russian terror does not have results.”

The summit at Ramstein is the 24th meeting of the Ukraine Defense Contact Group, but it’s only the second one Zelenskyy has attended in person.

A major shakeup this week of his war-time cabinet has raised questions at home and among Western partners alike. It included the dismissal of Foreign Minister Dmytro Kuleba, who had become the most familiar face of Ukraine’s foreign policy since the start of the war.

Even before the day of meetings got underway, Austin was the first of the allies to pledge additional support.

“I’m pleased to say that President Biden will announce today and additional $250 million security assistance package for Ukraine,” he said. “It will surge in more capabilities to meet Ukraine’s evolving requirements, and we’ll deliver them at the speed of war.”

The speed of the war is accelerating on both sides, however, and Zelenskyy has made it clear that his country can afford little time to wait.

Charlie D’Agata

Charlie D’Agata is CBS News’ senior national security correspondent. He was previously a senior foreign correspondent and has spent more than two decades covering international news for CBS.