Ukraine has received only limited aid this year from the United States -- which has provided tens of billions of dollars in military assistance to Kyiv -- because money to replace items drawn from US stocks ran out and opposition from some Republicans prevented the approval of supplemental funding.

The resulting gaps in assistance saw Kyiv's troops -- outnumbered and outgunned by Moscow's forces -- run short of key items such as artillery munitions, leaving them vulnerable.

"We're already seeing things on the battlefield begin to shift a bit... in Russia's favor. We're seeing them make incremental gains, we're seeing the Ukrainians be challenged in terms of holding the line," US Defense Secretary Lloyd Austin told lawmakers earlier this week.

The delay in approving new funding also created an opening for Russian President Vladimir Putin, who is "trying to exploit this time period where there is doubt created about the US's resolve," Austin said.

- 'Absolutely vital' -

Supplemental funding for Ukraine was stalled for months due to opposition from some hardline Republicans in the House of Representatives.

But lawmakers in the chamber on Saturday voted 311-112 to approve $61 billion for aid to Kyiv, with the Senate expected to quickly follow suit.

The Pentagon said it would move fast to get aid to Ukraine if it was approved by Congress.

"We are poised to respond quickly with a security assistance package" that would likely include "things like air defense and artillery capabilities," spokesman Major General Pat Ryder said.

Mark Cancian, a senior advisor at the Center for Strategic and International Studies, said the passage of new military aid funding "is absolutely vital for continued Ukrainian resistance."

As a result of the delay in approving it, "Ukraine has lost some territory, taken additional casualties, and been less able to fend off the air attacks," he said, noting that it has also "likely encouraged Putin to believe that he can outlast the West in a long war."

Ann Dailey, a policy researcher at the RAND Corporation, said Kyiv is "on the back foot," with its forces "desperately low on 155 artillery rounds" and "also struggling with air defense."

The shortage of 155mm rounds has left Ukraine in the position of "not being able to engage in counter-battery fires" against Russian artillery, she said.

- 'Untenable decisions' -

"If the enemy is firing artillery at you... and you don't have sufficient rounds to engage in counter-battery fires... it's extremely difficult, if not impossible, to stay in a defensive position -- you're likely going to have to displace," said Dailey, noting that this is what happened in Avdiivka, a town that Russia captured in February.

Air defense shortages meanwhile force Ukraine "to make untenable decisions about whether to sustain and protect their frontline to prevent further Russian advances or to protect their civilians and their industry," which are also being attacked by Moscow.

The approval of the additional funding will allow the United States to fulfill repeated pledges to back Ukraine for "as long as it takes," and officials including President Joe Biden say it will also directly benefit US industry.

"We'd be investing in America's industrial base, buying American products made by American workers, supporting jobs in nearly 40 states, and strengthening our own national security," Biden wrote in a Wall Street Journal op-ed this week, arguing for the approval of the aid funding.

In addition to helping Kyiv's forces resist Russia's invasion, American assistance also undermines a US adversary at a relatively small cost, Cancian said.

Military aid to Ukraine weakens Russia -- which the US has identified as an "acute threat" -- "without endangering any US forces and at a much lower cost than if US forces were engaged," he said.