The United States military has given backing to accusations against its British counterparts of carrying out air strikes that killed civilians in Iraq and Syria during the campaign against ISIS.
US military sources told the BBC there is “credible” evidence that civilians may have been harmed in Royal Air Force strikes.
It is an accusation that has been repeatedly denied by the UK’s Ministry of Defence.
In the UK’s five-year campaign against ISIS, the RAF has used more than 4,400 bombs and missiles against the extremists during a joint campaign with the US-led coalition.
A monitoring group has claimed that more than 40 civilians have died during air strikes by European nations.
Despite the RAF having being involved in some of these strikes, Britain has refuted claims it has been responsible for civilian deaths and to date has only accepted responsibility for just one civilian casualty.
The coalition as a whole has accepted responsibility for 1,300 civilian deaths – the majority due to US airstrikes.
But three strikes involving the British in Raqqa, Mosul and Bahrah, south of Deir Ezzor in Syria, between 2017 and 2018 reportedly killed 15 civilians.
In a 2018 strike in Mosul the MoD had said it was “highly likely” that those killed were Isis fighters but the US has told the broadcaster two civilians were “unintentionally” killed.
In August 2017 a strike in Raqqa against enemy fighters using a mortar system in a building left 12 civilians dead, the US said.
But the MoD has said it had seen “no evidence” of civilian casualties.
An attack on an ISIS fighter in Syria in January 2018 also left a civilian dead, the US says, in contrast to the British claiming the person was “very likely” to have been an extremist.
The coalition has used aerial footage to identify casualties and has faced criticism over its reliability.
The BBC said a senior US officer told it the RAF was “looking for certainty” of allegations of civilian harm, often when there was none.
The US military has a team of specialists who investigate allegations of civilian casualties and is required to publish details of civilian deaths.
In a statement, the MoD told The National: "The MoD examines all the evidence available to us, including a comprehensive assessment of all available mission data, and have seen nothing that indicates civilian casualties were caused.
“We always seek to minimise the risk of civilian casualties through our rigorous targeting processes, but that risk can never be removed entirely, particularly given the ruthless and inhumane behaviour of the adversary, including the deliberate use of human shields.”
The UK has previously come under fire for its monitoring of civilian deaths.
The 2016 Chilcot Report into the Iraq war had called on the government to do more to accurately track civilian casualties.
The MoD says it has killed 3,964 ISIS fighters and injured 298 to date.