Russia's war in Ukraine has been a "strategic failure in every way," US Secretary of State Antony Blinken said in a news conference on Tuesday.

Speaking alongside the Greek Foreign Minister in Athens, Blinken warned that allowing Russia's invasion of Ukraine to "go forward with impunity" would open up a "Pandora's box."

"One year after President Putin attacks Ukraine, it's clear that his war has been a strategic failure in every way," said Blinken.

"That's because of the courage of the Ukrainian people. But it's also because of the strength and unity of allies and partners around the world who have come to support Ukraine," he said.

The remarks followed US President Joe Biden's surprise visit to Ukraine Monday, which was hailed by Ukrainian President Volodymyr Zelensky as a "symbolic" visit from a "powerful ally."

Blinken commended Greece's "strong" and "outspoken" support for Ukraine and NATO, highlighting Greece's efforts to "strengthen the alliance's eastern flank" and facilitate shipments through the port of Alexandria.