The United States has agreed to sell $138 million worth of urgently needed military equipment to Ukraine to repair and upgrade its HAWK missile systems while a larger $60 billion aid package remains stalled in Congress.

Ukraine's air defenses, which largely consisted of Soviet-era planes and missile systems when Russia invaded in February 2022, have been vastly improved with US equipment and aid since the start of the war.

Washington has provided Ukraine with several advanced air defense systems to help counter Russian aerial attacks, including US-made Patriot missiles and older equipment such as the HAWK.

"Ukraine has an urgent need to increase its capabilities to defend against Russian missile strikes and the aerial capabilities of Russian forces," the US Defense Security Cooperation Agency said in a statement Tuesday.

"Maintaining and sustaining the HAWK missile system will enhance Ukraine's ability to defend its people and protect critical national infrastructure."

The sale would "not alter the basic military balance in the region," it added.

Ukraine has made increasingly urgent calls for the United States to start sending a proposed $60 billion aid package that has been blocked by Republicans in Congress for months.

The support package would allow the delivery of crucial weapons and ammunition to Ukrainian troops on the front lines.

With Russian forces ramping up attacks of late, Ukrainian President Volodymyr Zelensky said last Sunday that his country would lose the war if the US Congress did not approve the package.

Moscow launched in late March its largest aerial attack on Ukraine's energy infrastructure since the war began two years ago.

The attacks saw power cut to more than a million homes and took the country's largest hydroelectric plant offline.

Moscow has also stepped up attacks on Chasiv Yar as it seeks to build on territorial gains in recent months.

Chasiv Yar is less than 30 kilometres (18 miles) from Kramatorsk, an important rail and logistics hub for the Ukrainian army.

Washington has been Kyiv's key military backer since Russia invaded.

But with the new aid package held up by wrangling inside the Republican party over President Joe Biden's immigration policies, Ukrainian soldiers have been forced to ration ammunition amid uncertainty over when the next delivery might arrive.

The $60 billion package has already been approved by the Senate.