WASHINGTON, Feb 28 (Reuters) - The United States does not expect Russia to make significant territorial gains in Ukraine in the near-term, a senior Pentagon official said on Tuesday, describing the front lines in the year-long war as a "grinding slog."

"You may see small portions of territory change hands in the coming weeks and months. I do not think that there's anything I see that suggests the Russians can sweep across Ukraine and make significant territorial gains anytime in the next year or so," Colin Kahl, under secretary of defense for policy, told members of the House of Representatives.

Kahl made the remarks during a hearing focused on oversight of the nearly $32 billion in military aid President Joe Biden's administration has provided to Ukraine since Russia's invasion a year ago, including drones, long-range artillery systems, and air defense capabilities.

Some Republicans - whose party took control of the House in January - have expressed skepticism about the funds sent to the government of President Volodymyr Zelenskiy. Some party members allied closely with former President Donald Trump have called for an end to the aid, although party leaders in Congress back continuing support for Kyiv.

Republican-led House committees held two hearings on the aid on Tuesday, where administration officials described audits of the equipment and testified that it was not going astray.

"What we're not seeing is any evidence of significant diversion," Kahl said. "Our assessment is if some of these systems have been diverted, it's by Russians who have captured things on the battlefield, which always happens, but there's no evidence the Ukrainians are diverting it to the black market."

In October, the U.S. restarted on-site inspections in Ukraine to help keep track of the billions of dollars of weapons being supplied.

One thing the U.S. has not provided -- despite public appeals by Ukrainian officials -- is F-16 fighter jets.

Biden has ruled them out for now, and Kahl said it could take 18-24 months to deliver existing F-16s, with new ones taking months longer.

"It is a priority for the Ukrainians, but it is not one of their top three priorities," Kahl said.

Despite failing to take the capital city early in the war and many battlefield setbacks, Russia still controls about a fifth of Ukrainian territory. Kyiv has so far ruled out talks with Moscow and demanded that Russian troops withdraw to Ukraine's borders as of 1991 - the year the Soviet Union collapsed.

Russian forces on Tuesday pressed forward their weeks-long drive to encircle and capture the eastern Ukrainian city of Bakhmut where the commander of Ukraine's ground forces described the situation as "extremely tense."