William Hague has warned that the international community must be “prepared to do more” to end the Syria crisis following talks in Washington with US secretary of state John Kerry.
The Foreign Secretary said the two nations remained committed to securing a diplomatic solution but suggested they must be ready to increase pressure on Bashar Assad's regime.
Earlier, Prime Minister David Cameron stressed that no decision had been taken to arm the rebels as he announced he is to hold talks with Russian president Vladimir Putin in an effort to break the international deadlock on Syria.
Mr Hague described the atrocities being meted out on civilians as the "most urgent crisis anywhere in the world".
"We are both deeply concerned by what is happening to innocent people there," he told reporters at a press conference.
"The regime appears to be preparing new assaults, endangering the lives and safety of hundreds of thousands of Syrians who are already in desperate need.
"And the scale of the regime's oppression and the human suffering that it has caused beggars belief.
"The campaign of murder and tyranny that they have waged for more than 800 days now is not only a moral outrage, it's a grave threat to the wider region, it's a danger to our own national security."
"We agreed today that we cannot turn away from Syria and its people," he added.
Mr Kerry said the United States was still committed to peace talks and would "do everything we can" to bring about a transitional government.
He said: "Nobody wins in Syria the way things are going. The people lose and Syria as a country loses and what we are pushing for is a political solution that ends the violence, saves Syria, stops the killing and the destruction of an entire nation and that's what we are pushing for.
"So it's not a question to me whether or not the opposition can, quote, win, it's a question of whether or not we can get to this political solution."
He added: "There's a unanimity about the importance of trying to find a way to peace, not a way to war. The Assad regime is making that very difficult."
Mr Cameron will meet Mr Putin in Downing Street on Sunday ahead of next week's G8 summit in Northern Ireland.
"We should use the G8 to try and bring pressure on all sides to bring about what we all want, which is a peace conference, a peace process and a move towards a transitional government in Syria," he told MPs at Prime Minister's Questions.
Tensions with Moscow over the conflict in Syria ratcheted up last month when Britain combined with France to secure the lifting of the EU arms embargo blocking the supply of weapons to the rebels.
The Russians responded by announcing they would go ahead with the supply of sophisticated S-300 anti-aircraft missiles to the regime to deter "hotheads" from entering the conflict.
In the Commons, Mr Cameron strongly defended the ending of the EU embargo, saying it was designed to put pressure on the regime to attend planned peace talks in Geneva brokered by Russia and the United States.
"The point about lifting the arms embargo - which applied originally to both the regime and the official Syrian opposition - is to send a very clear message about our intentions and about our views to president Assad," he said.
"We all want to see a peace conference come about. The question is how are we most likely to put pressure on the parties to attend that peace conference.
"The Russian regime has been arming this regime for decades and frankly it is naive to believe anything else."
Pressed by Labour leader Ed Miliband, Mr Cameron stressed that no decision had been taken to arm the rebels, and he again indicated that MPs would be given a chance to vote if the Government did decide to go ahead.
"I have always believed in allowing the House of Commons a say on all of these issues," he said.