Russian forces have recaptured land hard won by Ukrainian troops at the peak of their summer counteroffensive in the south, just as Washington announced that it was releasing the last remaining Congress-approved package of military aid available to Kyiv.

The two developments highlight the war’s latest reality: With their counteroffensive stalled, Ukrainian troops are now on the back foot, struggling to contain Russian attacks all along the front line with dwindling resources.

Russia’s recent progress around the southern village of Robotyne illustrates those changing fortunes. Western-trained and -equipped Ukrainian brigades retook the village in August after weeks of fighting. But Russian forces, now attacking the area from the west and east, have regained some land on its flanks, threatening to further reverse Ukraine’s gains.

“Now, we are losing some fields, but if the U.S. aid is delayed, we will begin to lose towns,” Yehor Chernev, the deputy chairman of the Ukrainian Parliament’s committee on national security, defense and intelligence, said in an interview last week. “Without American ammunition, we are beginning to lose territory that was hard won this summer.”

For weeks, reluctance by Republican lawmakers in Congress to sustain assistance for Ukraine as the war stretches into another new year has thwarted Washington’s plans to send Kyiv more military aid. Congress declined again last week to pass a $50 billion security package for Ukraine, pushing back negotiations to next year.

Although some military aid could still flow from a separate program overseen by the Pentagon, the Biden administration is now tapping into the last remaining funds already approved by Congress. A $250 million package announced on Wednesday — which includes air defense equipment, artillery shells and over 15 million rounds of small arms ammunition — is likely the final tranche of available funding, according to American officials.

“When that one is done,” the National Security Council spokesman, John Kirby, told reporters last week, “we will have no more replenishment authority available to us.”

The Ukrainian military says its troops are facing shortages of critical equipment and ammunition. Some soldiers and commanders have said the shortfall has led them to scale back some operations and move to a defensive strategy.

The situation around the village of Robotyne, in the southern Zaporizhzhia region, may be a case in point. The Institute for the Study of War, a Washington-based research group, said on Wednesday that Russian forces had retaken positions captured by Ukraine during the counteroffensive, “likely after Ukrainian forces withdrew to more defensible positions near Robotyne for the winter.”

Russian troops have recently advanced from the southwest and east, pushing from Verbove, a nearby village that Ukrainian troops tried unsuccessfully to capture this summer to extend the bulge they had created in Russia’s defenses, according to the I.S.W. and open-source maps of the battlefield.

Russia’s advances have been limited so far: The open-source battlefield maps show that its forces have barely recaptured a few square miles of territory on the flanks of Robotyne. But Oleksandr Tarnavskyi, the head of Ukraine’s forces in the south, acknowledged to the BBC on Wednesday that “the situation in our sector is extremely difficult.”

Evgeny Balitsky, the Russian-appointed head of the part of the Zaporizhzhia region that Russia claimed to have annexed last year, told Russian television this week that he hoped Russian troops would soon retake Robotyne and reach the starting line of Ukraine’s counteroffensive.