U.S. Secretary of State Antony Blinken on Tuesday took the stage at a bar in Ukraine's capital to play guitar and sing Neil Young's 1989 hit "Rockin' in the Free World" with a local band.

The jam session came a day after Blinken spent a day meeting with senior officials, civil society figures and university students when he exhorted them against being discouraged in their ongoing fight against Russia. 

Blinken assured Ukrainians on his visit that they are not alone and that billions of dollars in American military aid on its way after months of political delays will make a "real difference" on the battlefield.

The performance, and a series of sunny comments from Blinken about Ukraine's battlefield prospects, was a startling juxtaposition to what analysts have called one of the most dangerous moments for Ukraine since Russia's full-scale invasion in February 2022.

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Blinken rocking out in Ukraine

U.S. Secretary of State Antony Blinken performs "Rockin' in the Free World" with members of The 1999 band at the Barman Dictat bar as he visits Kyiv, Ukraine, on May 14, 2024.  (BRENDAN SMIALOWSKI/Pool via REUTERS TPX IMAGES OF THE DAY)

Russian forces have taken swaths of territory along Ukraine's northeast border, and thousands of civilians in the Kharkiv region have fled the increasingly intense attacks.

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Blinken told Ukrainian leaders during his visit to Kyiv that despite a lengthy delay in U.S. military aid that left them vulnerable to these renewed Russian military strikes, more weaponry is coming and some has already arrived.

He made the case even as Ukrainian President Volodymyr Zelenskyy appealed to him personally for more air defense systems to protect civilians under intense Russian fire in the northeast. Blinken, on his fourth trip to Kyiv since the war began, also lambasted Russian President Vladimir Putin for underestimating Ukraine's determination to fight back.

Blinken posing with Zelenskyy in Ukraine

In this photo provided by the Ukrainian Presidential Press Office, Ukraine's President Volodymyr Zelenskyy, left, greets U.S. Secretary of State Antony Blinken, right, prior to their meeting in Kyiv, Ukraine, Tuesday, May 14, 2024. (Ukrainian Presidential Press Office via AP)

Despite his assurances, Moscow’s troops have captured around 40 to 50 square miles in recent days in the northeast Kharkiv region, including at least seven villages, according to open-source monitoring analysts. 

"We know this is a challenging time," Blinken told Zelenskyy after arriving on an overnight train from Poland. But, he added that U.S. military aid is "going to make a real difference against the ongoing Russian aggression on the battlefield."

Congress approved a long-delayed foreign assistance package last month that sets aside $60 billion in aid for Ukraine, much of which will go toward replenishing badly depleted artillery and air defense systems. Since then, the Biden administration has announced $1.4 billion in short-term military assistance and $6 billion in longer-term support.

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Zelenskyy thanked Blinken for the aid but said more is necessary, including two Patriot air defense systems urgently needed to protect Kharkiv.

The Associated Press contributed to this report.