US President Joe Biden promised new military aid for Ukraine worth $500 million during a surprise visit to Kyiv on Monday, almost a year to the day since Russia's invasion.

In a trip showing solidarity with Kyiv, Biden also said additional sanctions would be announced this week against the Russian elite and companies trying to evade sanctions to“back the Russian war machine”.

The military aid package will include artillery ammunition, anti-armour systems and air surveillance radars“to help protect the Ukrainian people from aerial bombardments,” he said.

"Freedom is priceless. It is worth fighting for as long as it takes. And that's how long we're going to be with you, Mr. President. As long as it takes,” Biden told President Volodymyr Zelenskiy after talks.

Biden appeared to make no mention of fighter jets, which Ukraine has been seeking from Western allies to help it push back the Russian forces.

Air raid sirens blared as Biden, 80, walked with Zelenskiy through central Kyiv but there were no reports of Russian missile or air strikes.

"The cost that Ukraine has had to pay is extraordinarily high. Sacrifices have been far too great,” Biden told reporters.

Describing the trip as “historic”, Ukrainian Foreign Minister Dmytro Kuleba said both Biden and Zelenskiy were pleased with their talks and that the visit sent a message of resolve to Russia, which he called “the swamp”.

"This visit is the victory of the Ukrainian people and President Zelenskiy... It is a clear signal to the swamp - no one is afraid of you!” he said.

Visiting Kyiv for the first time since Russia's invasion on February 24, 2022, Biden said Washington would stand with Ukraine as long as it takes.

He said his trip was intended to“reaffirm our unwavering and unflagging commitment to Ukraine's democracy, sovereignty, and territorial integrity.”

Biggest supplier of military assitance

Washington has been by far the largest supplier of military assistance to help Ukraine repel better-equipped Russian invaders. Kyiv expects a major new Russian offensive soon, and some military analysts say the offensive is already under way.

"This visit of the U.S. president to Ukraine, the first for 15 years, is the most important visit in the entire history of Ukraine-US relations,” Zelenskiy said.

Zelenskiy's chief of staff posted photographs of Biden in sun glasses walking side-by-side with Zelenskiy, who was wearing his trademark military-style clothing. Biden wore a blue and yellow striped tie.

In a speech, Biden commended Ukraine's courage during the war, adding: “I knew I would be back.”

The air raid sirens wailed while Zelenskiy and Biden were at the St Michael's Golden-Domed Cathedral on a square in central Kyiv where burnt-out Russian tanks have been placed.

Biden's trip fell on the day that Ukraine marks the deaths of more than 100 people - now known as the Heavenly Hundred - at anti-government protests that eventually toppled a Moscow-backed president in 2014.

After visiting the cathedral, Biden and Zelenskiy laid wreaths at the Wall of Memory for Ukrainian soldiers killed fighting Russia.

Several main roads in central Kyiv were closed off to traffic on Monday morning. Drivers stood waiting in traffic as gathering crowds of pedestrians peered over barricades to get a glimpse of who had come to the capital.