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.
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.
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."