Britain has agreed to deploy additional special forces in Syria alongside France to allow the US to withdraw its ground troops from the ongoing fight against the remaining Isis forces in the country.

US officials briefed on Tuesday that Britain and France would contribute 10% to 15% more elite soldiers, although the exact numbers involved remain secret.

The decision was first reported in the journal Foreign Policy, which described the development as “a major victory … for Donald Trump’s national security team” because few other countries had been willing to help out.

Earlier this week, it emerged that Germany had rejected a request to deploy ground troops in Syria. Other countries have been dragging their heels, the US admitted, although Italy is considering whether to join Britain and France.

The British support comes at a time when the UK is embroiled in a full-scale diplomatic row with Trump over Britain’s ambassador to Washington, Sir Kim Darroch, in the aftermath of leaked diplomatic cables in which the diplomat described the Trump administration as “inept” and “dysfunctional”.

Trump has been pushing to withdraw all ground troops from Syria, a decision that led to the resignation of his defense secretary, James Mattis, in December.

At the time, about 2,000 to 2,500 US troops were in the country. Trump then partially reversed the decision in February, meaning that about 400 would remain as a peacekeeping force for an indeterminate period.

The UK does not comment on the activities of its special forces, but it is well known that the SAS has been operating in the fight against Isis. Sources also confirmed the essence of the Foreign Policy report.

Concerns linger that, while Isis has lost all of the territory it once held, many of its fighters have gone to ground in northern Syria and Iraq. Too rapid a withdrawal could present an opportunity for Isis to strengthen and regroup.

The Royal Air Force is also part of an international coalition engaged in operations against Isis. Two Typhoons bombed six cave entrances 40 miles north-east of Tikrit in late April, from where it was believed some Isis members were holding out.

Earlier this week Theresa May praised “the brave men and women” who had been involved in the fight against Isis, known as Operation Shader.

In a speech at the UK’s military headquarters at Northwood in Hertfordshire, the outgoing prime minister said they had “helped destroy the territorial caliphate of Daesh, and continue the fight against the evil it stands for”.