A British drone did participate in an airstrike that Syrian authorities claim killed more than 60 government troops during the delicate ceasfire, an investigation found.
Theresa May said the UK would “never intentionally strike or focus on Syrian forces” after Britain admitted being part of a coalition air attack that allegedly killed dozens of Bashar Assad’s troops.
The Prime Minister said the UK would co-operate with the investigation by the US-led coalition into the attack on Saturday near the eastern city of Deir el-Zour.
The United States has accepted its aircraft may have accidentally hit a Syrian government position during an attack on Islamic State (IS), also known as Daesh, which is not covered by the ceasefire agreement.
Australia and Denmark have also said that their planes were involved in the strike.
It is understood Britain’s participation involved an unmanned RAF Reaper drone.
Speaking at the United Nations HQ in New York, Mrs May told broadcasters: “The UK was part of the coalition air strike that took place. We are now part of the investigation and it’s right that this is properly investigated.
“But, from the UK’s point of view we are there to deal with Daesh, to deal with the terrorist threat that is Daesh.
“We would never intentionally strike or focus on Syrian forces.
“But there is an investigation taking place and we will be contributing to that investigation fully.”
Russia - president Bashar Assad’s main foreign backer - said it had been told by the Syrian government that 62 of its troops had been killed in the strike.
The Syrian military said the coalition aircraft had hit a base which was surrounded by IS - enabling the militants to overrun their positions.
The incident lead to furious exchanges between Russia and the US at the UN, with the ambassadors of both countries storming out of an emergency session of the Security Council.
Vitali Churkin, the Russian ambassador who called the meeting, said it was suspicious the incident had happened just two days before US and Russian forces were supposed to begin co-ordinated strikes against the Islamists and that it had left a “very big question mark” over the peace deal.
US ambassador Samantha Power expressed regret for the loss of life but dismissed the summoning of the Security Council as a “stunt”, accusing Moscow of “moralism and grandstanding” that was “uniquely typical and hypocritical”.
The row erupted amid mounting reports of ceasefire violations - including air strikes by Syrian government forces on rebel held areas of the beleaguered city of Aleppo.