KYIV (Reuters) - U.S. Secretary of State Antony Blinken took to the stage with a Ukrainian band at a bar late on Tuesday during a surprise trip to Kyiv, drawing applause from the audience but some criticism online from people who felt the performance was "tone-deaf".

The mixed reaction reflected Ukrainians' recognition of the key role U.S. support plays in the war against Russia as well as growing concern among civilians over mounting battlefield losses and intensified Russian missile and drone attacks.

Blinken, who met President Volodymyr Zelenskiy during his visit and vowed unwavering U.S. support for Kyiv, referred to military difficulties in northeast Ukraine before he performed a rendition of Neil Young's "Rockin' in the Free World".

"Your soldiers, your citizens, particularly in the northeast in Kharkiv, are suffering tremendously," he told the audience at Barman Dictat in downtown Kyiv.

"But they need to know, you need to know, the United States is with you, so much of the world is with you. And they're fighting not just for a free Ukraine but for the free world, and the free world is with you too."

Despite those words, some Ukrainians said the performance, which was posted on social media for many to see, was off-key given the harsh reality of war that millions are living through.

"U.S. Secretary of State Antony Blinken's evening in Kyiv can be described in one word: inappropriateness," Svitlana Matvienko, executive director at the Agency for Legislative Initiatives, said on Facebook.

"With all respect to the allies, with all gratitude for the provided aid ... I was offended by this performance as a Ukrainian citizen whose loved ones are giving up everything so that we can resist."

The mood in Ukraine, where the war with Russia is now well into its third year, has turned increasingly pessimistic.

Russian forces are slowly advancing in the eastern Donbas region, attacking on a new front in the northeast and building up troops in the north. U.S. military aid was delayed for months and Kyiv desperately needs more air defences.

Serhiy Marchenko, a blogger with more than 34,000 followers on Facebook, said the performance did not resonate well.

"We are on a different wavelength. I would like to see more understanding from the allies," he wrote on the platform.

(Reporting by Olena Harmash and Simon Lewis; Editing by Gareth Jones)