The U.K. on Tuesday pledged an additional $620 million in new military supplies for Ukraine, including long-range missiles and four millions rounds of ammunition, at a time when Ukraine is struggling to hold off advancing Russian forces on the eastern front line of the war, now in its third year.

British Prime Minister Rishi Sunak spoke with Ukraine's President Volodymyr Zelenskyy on Tuesday morning to confirm the assistance and "assure him of the U.K.’s steadfast support for Ukraine’s defense against Russia’s brutal and expansionist ambitions," Sunak's office said.

Sunak was traveling to Warsaw later Tuesday to meet with Polish Prime Minister Donald Tusk and NATO Secretary General Jens Stoltenberg for talks about further aid for Ukraine.

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Ahead of the visit the U.K. government said Sunak would announce $620 million in new British military supplies, including 400 vehicles, 60 boats, 1,600 munitions and 4 million rounds of ammunition. The shipment will include British Storm Shadow long-range missiles, which have a range of some 150 miles and have proved effective at hitting Russian targets.

"President Zelenskyy thanked the Prime Minister for the U.K.’s continued support, saying the new military assistance would make a material difference to ordinary Ukrainians fighting on the front line to defend their country," Downing Street said.

The announcement comes three days after the U.S. House of Representatives approved $61 billion in aid for Ukraine, as American lawmakers raced to deliver a fresh round of U.S. support to the war-torn ally.

However, Downing Street did not indicate whether the aid would be immediately available for delivery. Zelenskyy has repeatedly pleaded for greater international assistance, warning that his country will lose the war without it.

Ammunition shortages linked to the aid holdup over the past six months have led Ukrainian military commanders to ration shells, a disadvantage that Russia seized on this year — taking the city of Avdiivka and currently inching towards the town of Chasiv Yar, also in the eastern Donetsk region.