Britain on Tuesday told Iran to stop supporting the Syrian government of Bashar al-Assad militarily, urging it to back efforts to broker peace in Syria instead.
Iran and its Lebanese Hezbollah have long backed Assad, and Western diplomats say Tehran is supplying weapons and personnel to help the Syrian leader win what is become an increasingly brutal war against rebel forces.
Addressing parliament on the eve of a Syrian peace conference in Switzerland, British Foreign Secretary William Hague said Iran's involvement in any future peace settlement would be "extremely important", saying it needed to urgently rethink its Syria policy.
"We want Iran to desist from support for the brutality of the Assad regime," Hague told lawmakers, calling an overnight report purportedly showing the systematic torture and killing of about 11,000 detainees "compelling and horrific".
"We will always try persuasion, but it is in the end in the interests of Iran for there to be peace in Syria and therefore we ask Iran to embrace that opportunity."
Hague spoke to Iranian Foreign Minister Javad Zarif on the phone after making his comments, telling him that the idea of brokering a transitional government by mutual consent was the way to end the conflict in Syria and that London remained open to working with Iran on that basis in future.
Britain's top diplomat told lawmakers that U.N. Secretary-General Ban Ki-moon had been right to withdraw an invitation to Iran to attend the Swiss peace talks because of Tehran's refusal to declare its support for such a transitional government.
"We were not opposed to Iran attending in principle but we wanted to see a clearer and more constructive public commitment by Iran to the objectives of the Geneva 2 conference," said Hague.
Hague was downbeat about the prospect of progress at the Swiss talks, saying they would be "formidably difficult".
"Realistically we are starting a process here," he said.
If the two sides could agree on confidence-building measures such as prisoner releases and local ceasefires, that would assist enormously, he said.
British diplomats say they want Iran to use its leverage with the Syrian government to press for greater humanitarian access and to help remove bureaucratic obstacles.
"We welcome Foreign Minister Zarif's stated desire for a peaceful solution without foreign interference. However, we are still waiting for his words to be translated into a change in Iran's approach on the ground," one diplomat said.