The US is preparing for the “final phase” in its war against the Islamic State in Syria, aimed at concentrations of Isis fighters in the Euphrates valley, senior administration officials have insisted, even though it is cutting $230m from its budget to stabilise areas of Syria captured from Isis.

They said on Friday that the cut from US funding had been more than compensated by $300m in extra contributions from Saudi Arabia, the United Arab Emirates and other US allies, and that the US remained committed to the “enduring defeat of Isis”.

The state department also unveiled a new push to make progress in political talks over Syria’s future, with the appointment of James Jeffrey, a former ambassador to Iraq, in the new position of special representative for Syrian engagement.

The announcements appeared to be aimed at reassuring allies of US staying power in Syria, while seeking to appease Donald Trump, who is anxious to end US involvement there. In March, the president ordered the stabilisation funds frozen after reading a news report about the planned spending, and declared in March that the 2,000 US troops in Syria would be leaving “very soon”.

After that declaration, Syrian forces, supported by Russia and Iran, swept away US-backed rebels in the south-west, while US allies in the north, the mainly Kurdish Syrian Democratic Forces (SDF), launched official talks with the Damascus regime, further undermining US diplomatic leverage.

State department officials at a briefing on Friday insisted that the US would stay the course in the continuing battle against Isis.

“We are remaining in Syria. The focus is the enduring defeat of Isis,” Brett McGurk, the special presidential envoy for the global coalition against Isis, said. “We have still not launched the final phase to defeat the physical Caliphate. That is actually being prepared now and that will come at a time of our choosing but it is coming.

“That will be a very significant military operation, because we have a significant number of Isis fighters holed up in a final area of the middle Euphrates valley,” McGurk said.

A report this month by the defence department inspector general said that at the end of June “Isis was estimated to still control about 5% of Syria and to have roughly 14,000 fighters in the country”.

The same report gave a top estimate of the number of Isis fighters in neighbouring Iraq as up to 17,100, while cautioning that all such estimates are subject to wide margins of error.

“Politically there has been a lot of messaging going on for a few weeks now, aimed towards the president specifically, to push this idea: the battle is far from over, that stabilisation is crucially important because these are the threats that remain,” Charles Lister, a Syria expert at the Middle East Institute, said.

“Clearly the message coming from the defence department is that the struggle is significant, and far beyond this final battle that Brett’s talking about, and that obviously puts more of an onus of responsibility on the stabilisation mission,” Lister added.

After reading a report in March that the US was planning to spend over $200m more on stabilisation projects in Syria, in areas captured from Isis, an angry Trump is reported to have ordered the spending frozen. At a rally in Ohio, he told the crowd: “We’re knocking the hell out of Isis. We’ll be coming out of Syria, like, very soon. Let the other people take care of it now.”

In an effort to maintain the stabilisation effort, focused on making liberated areas safe and habitable for residents to return, the secretary of state, Mike Pompeo, held a meeting of 54 partners and allies on the margins of the Nato summit in July, to canvass contributions. Saudi Arabia responded with a $100m contribution, the UAE with $50m, and other allies offered another $50m.

McGurk said that despite the switch in funding, the US would remain largely in control of stabilisation efforts in Syria.

“In Syria we very much have the overall lead in stabilisation, because we have a team on the ground,” McGurk said. “A significant proportion of these contributions are coming directly into the US account which oversees the overall stabilisation in Syria.

“We think the way we’ve organised it is pretty good. It maintains US leadership of the coalition, which has been a success, but the emphasis is on burden-sharing from other partners, and the partners have really stepped up.”

David Satterfield, the acting assistant secretary for near eastern affairs, added: “What we have done … is we have mobilised the critical international support that the president very much wanted to see and which the international community has responded to quite positively.”

With the declarations of renewed commitment to the counter-Isis campaign and the appointment of Ambassador Jeffrey to concentrate on political talks, the Trump administration is hoping to boost its leverage with Russia in negotiations over Syria’s political future. The Assad regime has largely ignored those talks while making a series of military advances, with Russian and Iranian support.

“We have been very clear – as clear as it is possible to be – with the government of Russia that there will be no international reconstruction assistance for Syria without the irreversible political process validated by the UN,” Satterfield said. “There should be no ambiguity about that.”