The US will buy 100,000 rounds of howitzer artillery from South Korean manufacturers to provide to Ukraine, a US official has said.
It is a deal the two governments have been working on for some time.
The agreement comes as Ukrainian leaders press for more weapons and aid to take advantage of a counter-offensive pushing Russian forces out of some areas they took over earlier in the war.
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This will reportedly relieve concerns within the US military — particularly the army and the marine corps — with reports they are worried that persistent transfers of the Pentagon's howitzer ammunition to Ukraine are eating into their own stockpiles.
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The US Department of Defense (DOD) recently also announced further Ukraine security assistance valued at up to $400m to "meet Ukraine's critical security and defence needs".
Other defence officials confirmed the broad outlines of the contract and said it will help with stockpile pressures, specifically involving the howitzer ammunition, which Ukrainian forces have been using at a high rate.
Last week, a defence official briefing reporters said Ukraine was burning through as many as 7,000 rounds of ammunition a day, while Russia was firing as much as 20,000 rounds daily.
Former NATO commander Richard Shirreff told Forces News that Ukraine "will achieve their military objectives" so long as the West continues to provide the support necessary "and indeed ramps it up".
South Korea's defence ministry in a statement acknowledged ongoing talks over exporting an unspecified number of 155mm artillery shells to shore up diminishing US inventories.
However, the ministry said the negotiations were proceeding under the presumption the US will be the "end user" of those rounds and that Seoul maintains its principle of providing only non-lethal support to Ukraine.
The South Korea agreement provides a sharp counterpoint to US accusations earlier this month that North Korea covertly shipped artillery to Russia.
North Korea has aligned with Russia over the war in Ukraine while also blaming the US for the crisis, insisting the West's "hegemonic policy" forced Russia to take military action to protect its security interests.
However, Pyongyang has repeatedly denied US claims it has been sending large supplies of artillery shells and other ammunition to Russia, accusing Joe Biden's administration of a smear campaign.
Experts say North Korea has the potential to become a major source of munitions for Russia, considering the interoperability of their weapons systems based on Soviet roots.
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Until now, South Korea had previously limited its support for Ukraine to non-lethal equipment and supplies.
Seoul's defence ministry confirmed at the time it had rejected the Ukrainian request for anti-aircraft weapons, citing the South Korean government's principle of sending only non-lethal aid.
International security experts have said both North Korea and South Korea maintain vast stockpiles of ammunition due to the decades-long tensions along their heavily fortified and militarised shared border.
In a statement, US Army Lt Col Marty Meiners, a Pentagon spokesman, said the US government has been in discussions to buy ammunition from South Korea's non-government defence industrial base.
The ammunition would not come from South Korean military stocks. He declined to provide details.
Mr Meiners said any potential sales always take into account the South Korean military's readiness and requirements and "will not detract from our defensive posture or readiness to respond against regional threats".