US officials are increasingly critical of Ukraine’s counteroffensive strategy and gloomy about its prospect of success, deepening tensions between Kyiv and Washington at the most critical point in the war since Russia’s full-scale invasion.

Kyiv launched its counteroffensive against Russian occupying forces earlier in the summer with staunch US backing, but progress has been slow and analysts say Ukraine will struggle to recapture significant territory before muddy ground hinders manoeuvres or its forces run out of combat power.

The pessimism about the counteroffensive in Washington comes just weeks before a $43bn package of US funding for Ukraine is set to expire, requiring the Biden administration to secure congressional approval for more assistance for the country.

“I don’t think that you’ll hear an argument from anyone that this is going well right now or that this is heading to a place that people would view as good, but there is not much by way of plan B,” said Samuel Charap, a senior political scientist at the Rand Corporation.

The US and Ukraine originally planned for a spring offensive that would quickly roll back Russian occupying forces in the summer. But slow progress on the ground has prompted Kyiv to revert to more traditional tactics rather than the combined arms manoeuvres the US and its western allies taught them in Europe earlier this year.

“We’re doing everything we can to support Ukraine in its counteroffensive,” said Jake Sullivan, US national security adviser, on Friday. “We’re not going to handicap the outcome. We’re not going to predict what’s going to happen because this war has been inherently unpredictable.”

Ukrainians continued to make some small gains this week, including liberating the village of Urozhaine. But US officials are privately girding for what increasingly looks like a war of attrition that will last well into next year, while publicly reiterating continued support for Kyiv.

“It’s been about 10km of advance at most no matter where you look in this offensive,” said Michael Kofman, a senior fellow at the Carnegie Endowment for International Peace and principal research scientist at the Center for Naval Analyses.

One point of tension between US and Ukrainian officials has centred on how Kyiv has deployed its military. US officials have encouraged Ukraine to be less risk-averse and fully commit its forces to the main axis of the counteroffensive in the south so it would have a chance of breaking through Russian lines to reach the Sea of Azov, effectively cutting Russia’s land bridge in southern Ukraine to Crimea, a critical military hub.

Washington has also urged Ukraine to send more combat power to the south, and stop concentrating on the east, where almost half of its forces are engaged. But Ukraine has instead deployed some of its best fighting units in eastern Ukraine in a battle to recapture Bakhmut.

Officials in Kyiv including President Volodymyr Zelenskyy and some of the Biden administration’s critics have called for the west to offer Ukraine heavy weaponry, and say the counteroffensive’s progress will remain slow unless Washington sends more long-range fire and air power to support it.

But US officials say the US does not make enough of the tactical ballistic missiles to supply the numbers that would make a significant difference on the battlefield. They have also said they are holding back the advanced long-range missiles sought by Kyiv because of concerns that their supply could escalate the conflict with Russia.

Some analysts say Kyiv’s focus on long-range weaponry is misplaced, given its modest impact in a war increasingly fought with artillery, including the cluster munitions the US recently sent to Ukraine to make up for dwindling supplies of other munitions.

“There’s no magic wands,” Charap said. “It’s hard to make the case that long-range strike [missiles] can fix the problem of minefields or all these defences.”

He added: “It will complicate Russian logistics but that’s not the main or the only problem the Ukrainians are facing today.”

US President Joe Biden included planned funding of another $13bn in lethal aid for Ukraine in a supplemental budget request to Congress earlier this month. The money would last until the end of the year.

But the extra funding faces a tricky path to passage on Capitol Hill amid a broader fight about government spending levels that could even lead to a shutdown of federal operations as soon as October.

“Blockage or reduction is unlikely, but a political battle is inevitable, given rising concerns on both the left and the right,” Mark Cancian, a senior adviser at the CSIS, a Washington think-tank, wrote in a note this week.

“So far however the opposition has not stopped or even reduced aid in the face of strong bipartisan support. What is new is the disappointing results of the Ukrainian counteroffensive so far.”

Even if Congress authorises the latest package of Ukraine funding requested by the White House, some US officials and analysts say it is unlikely that Washington will be able to offer the same level of lethal assistance to Ukraine next year, given the looming presidential election and munitions manufacturers’ longer-term schedule to increase production.

While Biden has remained stalwart in his support for Ukraine, Donald Trump, his predecessor and the frontrunner for the Republican presidential nomination, has pledged to end the war immediately if he is elected, while other Republican contenders for the 2024 nomination have expressed ambivalence.

Ron DeSantis, the Florida governor, walked back a comment from earlier this year in which he dismissed the war as a “territorial dispute”, but speaks little about Ukraine on the campaign trail. Vivek Ramaswamy, the anti-ESG entrepreneur who has been rising in national polls, suggested that the US should force Kyiv to strike an agreement with Vladimir Putin.

According to a CNN poll released this month, 55 per cent of Americans now oppose more funding for Ukraine from Congress, while 45 per cent support it, a big reversal from overwhelming backing for Kyiv early in the conflict.

Republican congressman Andy Harris, a co-chair of the Ukraine Caucus, had been a steadfast supporter of Kyiv’s effort, but told a town-hall meeting in Maryland this week that the counteroffensive had “failed”, saying aid for Ukraine should now be unwound.

“Is this more [of] a stalemate? Should we be realistic about it? I think we probably should,” Harris told constituents in Abingdon, Maryland, 25 miles north-east of Baltimore. “I’ll be blunt, it’s failed.”

He was also pessimistic about the course of the war, saying, “I’m not sure it’s winnable anymore.”

While no one in the Biden administration has ruled out a breakthrough before winter sets in, few are projecting much optimism.

“I think the jury is still out,” secretary of state Antony Blinken told The Washington Post recently.

“I don’t think we will know for . . . at least a month and maybe longer whether the counteroffensive is going to make significant strategic gains for Ukraine in terms of recovering territory.”

Additional reporting by James Politi in Washington