Defense Secretary Lloyd J. Austin III made an unannounced visit to Kyiv on Monday to bolster support for the war effort as Ukrainian forces steadily lose ground to Russian troops and allies stop short of fully endorsing Ukraine’s latest plan to end the conflict.

The visit by Mr. Austin, his third to Ukraine since Russia’s full-scale invasion began in February 2022, comes three days after President Biden met with allied leaders in Germany to rally support for Ukraine, and with the fate of future American military aid to the country hanging in the balance of the U.S. presidential election in two weeks.

Mr. Austin, who arrived by train from Poland, is set to meet with President Volodymyr Zelensky and top Ukrainian military leaders to discuss how to continue backing Ukraine’s military campaign against a growing set of challenges.

In recent days, Russian troops have clawed back much of the territory Ukraine seized in Russia’s western Kursk region. The Russians are also making slow, grinding progress in the Donbas region in Ukraine’s east despite staggering casualties. And Moscow has sharply increased its drone attacks across Ukraine.

“The United States and our allies and partners are determined to stand by Ukraine for the long haul,” Mr. Austin said last week at a meeting of NATO defense ministers in Brussels. The United States has provided Ukraine more than $61 billion in security aid since the start of the war.

Mr. Austin also said U.S. officials would help Ukraine train and equip new units it is building. Reports have surfaced that many Ukrainian units fighting in southern Donetsk and other front-line areas are undermanned.