US Secretary of State Antony Blinken told President Volodmyr Zelensky during a trip to Kyiv that part of a major US aid package had arrived in Ukraine and that more was on its way.
Blinken's trip is the first by a senior US official since Congress passed a long-delayed $61 billion aid package last month. Kyiv's outmanned troops are battling a new Russian offensive in the northeast as well as assaults in the east.
"We know this is a challenging time. But we also know that in the near term the assistance is now on the way, some of it has already arrived and more of it will be arriving," Blinken said.
"And that's going to make a real difference against the ongoing Russian aggression on the battlefield."
Ukraine recaptured swathes of territory during the first year after Russia's invasion in 2022, but a Ukrainian counter-offensive faltered last year and in recent months Russia has retaken the initiative at the frontline.
Military aid from Washington, Kyiv's main backer, was held up for months, blocked by Republicans in the U.S. Congress until they finally allowed a vote last month, when it passed with support from both parties.
Zelenskiy, addressing Blinken in English, said air defence supplies were, "the biggest deficit for us" with Russia conducting long-range aerial attacks since March that have pounded electricity facilities and caused blackouts.
"Really we need today two Patriots for Kharkiv, for Kharkiv region because there the people are under attack. Civilians, warriors, everybody they are under Russian missiles."
Blinken arrived in Kyiv by train early on Tuesday morning on the previously undisclosed visit, which comes days after Russia launched a ground incursion into the north of the region of Kharkiv, opening a new front and stretching Ukraine's soldiers.
Russia now controls about 18 per cent of Ukraine and has been gaining ground since the failure of Kyiv's 2023 counter-offensive to make serious inroads against Russian troops dug in behind deep minefields.

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