Washington (CNN) -- President Barack Obama was cautious on Monday about whether the international community could broker peace in Syria, while British Prime Minister David Cameron applied new urgency for diplomacy, saying the war-wracked country's history is being "written in the blood of her people."
Obama and Cameron appeared at a joint news conference at the White House where questions about Syria touched on accelerating peace efforts and whether Russia, a close ally of Damascus, would reverse course and put pressure on the regime of Bashar al-Assad to abandon power.
Obama said the political situation is complex and identifying any solution satisfactory to the groups comprising Syria's opposition and finding common ground on the scope and timing of transition away from al-Assad would be challenging.
"Frankly, sometimes once ... the furies have been unleashed in a situation like we're seeing in Syria, it's very hard to put things back together," he said.
Cameron said the British government had amended the European Union's Syrian arms embargo to allow for Britain to provide increased "technical assistance and technical advice" to the opposition. He also said plans are under way to double Britain's "nonlethal support to the Syrian opposition in the coming year."
"Syria's history is being written in the blood of her people and it is happening on our watch," Cameron said. "The world urgently needs to come together to bring the killing to an end. None of us have any interest in seeing more lives lost, in seeing chemical weapons used, or extremist violence spreading even further."
The two-year civil war in Syria has claimed more than 70,000 lives, according to U.N. estimates.
The United States recently boosted aid to Syrian rebels but pressure is growing for an accelerated and more muscular western response to the crisis following the American disclosure in recent weeks that it believes chemical weapons were used in Syria.
Peace proponents are focusing closely on Moscow and whether Russian President Vladimir Putin would join an international push for al-Assad to step aside.
Both Obama and Cameron said they had spoken with Putin about Syria.
Obama pointed to what he called "lingering suspicions" between Russia and the West, which have influenced Putin's decision to so far not join the United States and Britain in leading the search for a political transition in Syria.
"I've spoken to President Putin several times on this topic, and our basic argument is that as a leader on the world stage, Russia has an interest as well as an obligation to try to resolve this issue in a way that can lead to the kind of outcome that we'd all like to see over the long term," Obama said.
For instance, Obama said preventing the region from becoming a safe haven for extremists is in the global interest.
Cameron described a "frank conversation" with Putin on the issue last week while in Russia and welcomed his agreement that the violence must end. How that is achieved is still an open question.
Russia so far has rejected U.N. resolutions condemning the Syrian government crackdown on opposition groups. But it has joined with the United States in trying to organize an international peace conference.
Cameron admitted that Putin also thus far has "taken a different point of view" on al-Assad's future.
"I think both the Russian president, the American president, myself, I think we can all see that the current trajectory of how things are going is not actually in anybody's interests and so it is worth this major diplomatic effort, which we are all together leading, this major diplomatic effort to bring the parties to the table to achieve a transition at the top in Syria so that we can make the change that country needs," Cameron said.
He acknowledged that defining what that transition might look like and what it might take to achieve remains a "formidable" challenge.