The transfer last week came as Ukraine suffers from significant shortages of ammunition and US Republican lawmakers block new aid funding, but it does not address Kyiv's need for key items such as artillery and air defense munitions.

"The US government transferred over 5,000 AK-47s, machine guns, sniper rifles, RPG-7s and over 500,000 rounds of 7.62mm ammunition to the Ukrainian armed forces" on Thursday, the US Central Command (CENTCOM) said on social media.

"These weapons will help Ukraine defend against Russia's invasion" and are enough material to equip a brigade, it said.

The arms and ammunition were seized between May 2021 and February 2023 from four "stateless vessels" as the supplies were being transferred from Iran's Revolutionary Guards to Yemen's Huthi rebels, CENTCOM said.

"The government obtained ownership of these munitions on December 1, 2023, through the Department of Justice's civil forfeiture claims," it said.

The Huthis have been targeting vessels in the Red Sea and Gulf of Aden since November 2023 in attacks they say are in solidarity with Palestinians in Gaza -- a significant international security challenge that threatens a major shipping lane.

- Congressional impasse -

"Iran's support for armed groups threatens international and regional security, our forces, diplomatic personnel, and citizens in the region, as well as those of our partners. We will continue to do whatever we can to shed light on and stop Iran's destabilizing activities," CENTCOM said.

Washington made a similar transfer to Ukraine in early October, providing 1.1 million rounds of 7.62mm ammunition that was seized from Iranian forces on the way to Yemen.

But funding for crucial artillery and air defense munitions for Ukraine has been held up by Republican lawmakers who have stalled a $60 billion support package in the US Congress since last year.

US Defense Secretary Lloyd Austin testified during a Senate hearing Tuesday that Washington not living up to its commitment to Kyiv would encourage America's foes.

"It would be a signal that the United States is an unreliable partner, and that would encourage and embolden autocrats around the globe to do the types of things that (Russian President Vladimir) Putin has done," Austin said.

The United States announced a $300 million assistance package for Kyiv on March 12 -- the first since December -- that included anti-aircraft and anti-tank weapons and artillery shells, but warned at the time that it would run out after a few weeks.

That package was funded by using money that the Pentagon saved on other purchases, allowing the US government to provide aid despite the congressional impasse.

US officials have spearheaded the push for international support for Ukraine, quickly forging a coalition to back Kyiv after Russia invaded in 2022 and coordinating aid from dozens of countries.

Washington has been by far Kyiv's biggest donor of security aid, committing tens of billions of dollars to aid Ukraine since the invasion.