Ukraine’s allies fear military support for its battle against Russia is nearing a peak, with senior European officials increasingly concerned about the flow of aid next year as the US enters a divisive presidential campaign.

Washington has been Ukraine’s dominant source of weaponry and US officials say sufficient preapproved funds remain to sustain Kyiv for about five more months, covering a crucial counter-offensive planned for the coming weeks.

But European allies are increasingly uncertain about whether the US will come close to matching its existing $48bn package, adopted in 2022, particularly as it requires a vote in Congress this autumn against the backdrop of more partisan debate on the war.

“No one knows,” said a European official, one of 10 senior figures in countries allied with Ukraine who spoke to the Financial Times about the next phase of the war. “We can’t keep the same level of assistance forever,” the senior official added, arguing the current rate of support could be sustained for a year or possibly two but not more.

US president Joe Biden, who has cast the war in Ukraine as a strategic defeat for Russia, has given no indication he intends to reduce support. But he is preparing for a re-election campaign with Donald Trump, a critic of open-ended US backing for the war, his most likely Republican rival in the 2024 race.

Biden will meet his G7 counterparts in Hiroshima, Japan, on Friday for a three-day summit where reaffirming western support for Ukraine will be one of the key goals. “President Biden has been very clear about supporting Ukraine for as long as it takes,” a White House official said. “We continue to see strong unity in support of Ukraine from our allies . . . We expect to see that unity on full display at the G7.”

Trump declined to say whether he wanted Ukraine to prevail in the war during a CNN town hall last week. He went on to demand Europe step up contributions and said he would arrange talks between Russia’s leader Vladimir Putin and Ukraine’s president Volodymyr Zelenskyy to rapidly end the conflict.

“What Donald Trump says has a lot of impact on how difficult this issue becomes in Congress,” said US Democratic Senator Chris Murphy, a member of the Senate foreign relations committee. “His position on Ukraine funding will have a lot to do with what happens if we need to reauthorise support.”

Some allies in regular contact with the US over Ukraine say Washington sees the next five months as critical to the outcome of the conflict and the last real chance for Kyiv to change the situation on the ground.

With polling showing US support for Ukraine waning, some European allies say the Biden administration is under pressure to show that tens of billions of dollars in assistance have made a significant impact on the battlefield.

“It is important for America to sell this war as a successful one, also for domestic purposes to prove that all of those aid packages have been successful in terms of Ukrainian advances,” a second European official said.

Some of the officials pointed to the UN General Assembly and G20 leaders’ summit taking place consecutively in early September as two crucial diplomatic events where both sides would come under large pressure to come to the table.

“If we get to September and Ukraine has not made significant gains, then the international pressure on [the west] to bring them to negotiations will be enormous,” said one of the officials on condition of anonymity. “The same is true for Russia if the counter-offensive leaves them routed.”

But western officials are also hopeful that Ukraine’s counter-offensive, backed by unprecedented supplies of Nato-standard weapons, will deliver major gains that they could force Putin to negotiate peace terms of some form.

Some Biden administration officials see that as Kyiv’s opportunity to open talks. “The message [to Kyiv] is basically that this is the best you’re going to get,” said another European official, referring to the level of western military support. “There’s no more flexibility in the US budget to keep writing cheques, and European arms factories are running at full capacity.”

Biden administration officials say the US has demonstrated widespread bipartisan support for Ukraine and that future assistance should not be in doubt. They pointed to recent comments by House Majority Leader Kevin McCarthy who said he supports assistance for Ukraine and “will continue to support [it].” However, he has previously said there will be “no blank cheque” for aid to Ukraine.

Germany, France and the UK all made significant contributions to Ukraine this week during surprise visits by Zelenskyy. But the US plays a vital role in setting the scale and tempo of contributions.

Diplomats have begun looking at how the political calendar in the US may affect the timing of decisions on Ukraine, especially given Washington will host a Nato summit in the summer of 2024, months before the presidential elections.

US military assessments say Ukraine is unlikely to be able to achieve all its political goals on the battlefield this year, even if it does make gains during the counteroffensive. Ukrainian and European officials also acknowledge this view privately.

“We can all agree that we would like to be somewhere else next year, something which is closer to peace, closer to the restoration of Ukrainian stability and sovereignty,” a senior European official said.