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

Biden made an unannounced trip to Kiev on Monday where he 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.

"The cost that Ukraine has had to pay is extraordinarily high. Sacrifices have been far too great," Biden told reporters in Kiev, where he held talks with President Volodymyr Zelenskyy.

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

Air raid sirens blared as Biden, 80, walked with Zelenskyy through central Kiev but there were no reports of Russian missiles or air strikes.

Visiting Kiev for the first time since the start of Russia's Ukraine offensive, Biden said Washington would stand with Ukraine as long as it takes.

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

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Largest supplier of military assistance

The United States has been by far the largest supplier of military assistance to help Ukraine repel better-equipped Russians.

"This visit of the US president to Ukraine, the first for fifteen years, is the most important visit in the entire history of Ukraine-US relations," Zelenskyy said.

Zelenskyy's chief of staff posted photographs of Biden in sunglasses walking side-by-side with Zelenskyy, who was wearing his trademark military-style clothing.

Kiev is preparing for a possible major new Russian offensive that some military analysts say is already underway in the east.

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

The air raid sirens wailed while Zelenskyy and Biden were inside the St Michael's Golden-Domed Cathedral on a square in central Kiev 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.

Several main roads in central Kiev 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.

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