KYIV, Ukraine (AP) — Ukrainian commander Oleksiy Tarasenko witnessed a frightening shift last month in Russia's efforts to punch through Kyiv’s defense of the industrial region known as the Donbas.

Standing against Russia’s unyielding advance in the strategic front-line town of Chasiv Yar, he noticed that, instead of making typical light infantry assaults, Moscow’s forces were taking brazen risks by launching battalion- and platoon-sized attacks, sometimes with up to 10 combat vehicles.

His men destroyed up to 80 tanks in the weeks that followed, but it did not slow the enemy. The confidence of the Russian military reflected the Kremlin's knowledge that Ukraine’s ammunition supplies were dwindling as the U.S. dawdled over approving more military aid.

Saturday’s passage by the U.S. House of Representatives of a much-awaited $61 billion package for Ukraine puts the country a step closer to an infusion of new firepower that will be rushed to the front line to fight Moscow’s latest attacks. But the clock is ticking, with Russia using all its might to achieve its most significant gains since its invasion by a May 9 deadline, when Moscow commemorates World War II Victory Day. In the meantime, Kyiv has no choice but to wait for replenishment.

Seeing a window of opportunity, Russia has seized the momentum on the battlefield and forced Kyiv’s forces to cede tactically significant territory, one painful meter (yard) after another.

Wave after wave of mechanized units came for Tarasenko's brigade. Protected under an umbrella of attack drones and artillery fire, they reached the foot of Chasiv Yar, which is the gateway to Ukraine’s defensive backbone in the Donetsk region.

“They concentrated disproportionately enormous resources in this direction,” said Tarasenko, deputy commander of the 5th Separate Assault Brigade. “The most difficult thing is to cope with this constant onslaught from the enemy, which does not change, even though the enemy is losing a lot of military equipment and soldiers.”

The Pentagon has said it could get weapons moving to Ukraine within days if the Senate and President Joe Biden give final approval to the aid package. But experts and Ukrainian lawmakers said it could take weeks for the assistance to reach troops, giving Russia more time to degrade Ukrainian defenses.

The seven-month effort to pass the package effectively held Ukraine hostage to the internal politics of its biggest ally. It also raised concerns about how the shifting sands of American politics will influence future military support.

European partners cannot match the volume and scope of American assistance, which remains Kyiv's main hope to win the war. But that support has come with red lines, including rules that forbid using Western-supplied weapons for strikes inside the Russian Federation. Some Ukrainian officials argue that such limits handicap their ability to cripple the enemy's more robust capabilities.

Assuming the assistance arrives in the next two months, plans are afoot for a potential late-summer offensive. Analysts have argued that future support should not count on one big decisive battle, but a sustained strategy over many years.

But first, Ukraine must hold off Russia's attempts to break defensive lines and entrenched positions.

In the past month, The Associated Press spoke to a dozen commanders across the active zones of the eastern front line, from Kupiansk in the northeast to Bakhmut farther south. They said their soldiers have rationed shells and struggled to repel enemy attacks with insufficient artillery ammunition.

They are also running critically low on air-defense missiles, not only for high-end Patriot systems that protect cities, but also for tactical air systems. That has given Russian fighter-bombers an opportunity to lob thousands of deadly aerial glide bombs against Ukrainian positions, razing defenses to the ground, something Russia’s air force has not been able to do before.

Since January, the Kremlin has seized 360 square kilometers (140 square miles) of Ukrainian territory, roughly the size of the American city of Detroit, according to the Washington-based Institute for the Study of War.

Ukrainian commanders have complained about dire ammunition shortages since late December. By February, heads of artillery units in several regions said they had less than 10% of the supplies they needed as Kyiv rushed to economize shells.

Nowhere are supplies more needed than in Chasiv Yar, where after weeks of fierce fighting, Moscow is intent on conquering the town. Ukraine’s commander in chief, Oleksandr Syrski, said Russia’s top military leadership ordered its soldiers to capture the town by May 9, Russia’s Victory Day, a holiday that marks the defeat of Nazi Germany.

To reach that goal, Russia unleashes daily drone assaults and glide bombs on Ukrainian forces that have no way to counterattack.

Time is of the essence, said Yurii Fedorenko, commander of the Achilles battalion of the 92nd brigade in the Chasiv Yar region.

“They simply destroyed our positions with massive strikes. Now those positions are constantly hit by artillery, making it impossible to recapture them," he said.

“Now we have nothing to answer the enemy with,” he added.

Commanding men who have reached extreme levels of burnout, Fedorenko acknowledged the Russians were steadily advancing. At the time of the interview, Russian forces were just 500 meters (1,640 feet) from the town, he said.

The soldiers who died to protect land that was lost could have been spared if the U.S. aid had been approved earlier, he said.

“Our losses could be reduced to a minimum, and we would not have lost territories that would later have to be reconquered.”

Russia picked up momentum soon after gaining control of Avdiivka in February. Immediately, Moscow’s troops sought to reinforce their tactical success and push further into larger, strategically significant towns — Kostiantynivka, Sloviansk and Druzkhivka — that together form the fortress wall of Ukraine’s main defense of the Donetsk region.

A win in Chasiv Yar, which had a prewar population of 12,000, would bring Russia one step closer to breaking that barricade.

“If the Russians manage to take Chasiv Yar, they are only about 5 to 7 kilometers away from the southernmost link in that chain,” said George Barros, an analyst at the Institute for the Study of War. If Russia manages to push into the seam between Kostiantynivka and Druzhkivka, it would be able to attack the fortress belt, he said.

“Then we get into the territory where the Russians might be making some really substantial operational gains and eroding Ukraine’s ability to defend the rest of Donetsk,” he said.

An injection of new supplies would give Ukrainian forces cover and help them push the enemy back. But Russia will continue to have the upper hand in both manpower and ammunition. The Russian military has the ability to generate 20,000 to 30,000 new volunteers per month, and it holds a roughly 6-to-1 advantage in artillery.

Until now, that reality has precluded any potential for a Ukrainian counteroffensive.

Russian fighters “do not have the feeling that they will now lose some critical armored vehicle unit or soldier unit for which they will no longer have new reinforcements,” Tarasenko said. “They don’t worry about it. That is their advantage.”