Obama, Cameron raise pressure on Russia to help end Syrian conflict

President Obama and British Prime Minister David Cameron turned up the pressure on Russia Monday, saying Moscow has an "obligation" to back U.S.-led efforts to end the Syrian civil war.

"As a leader on the world stage, Russia has an interest as well as an obligation to try to resolve [Syria] in a way that can lead to the kind of outcome that we'd all like to see over the long term," Obama said during a joint press conference with the British leader at the White House.

Over 80,000 have been killed since opposition forces began battling to oust Syrian President Bashar al-Assad three years ago, leading to a humanitarian crisis and growing congressional calls for the U.S. to arm rebel groups.

The Obama administration has placed a priority on diplomatic means to end the violence, but efforts to tighten the noose around the Assad regime have been stymied by Damascus’s allies, Russia and China.

Cameron warned of the humanitarian crisis after three years of war, and urged the West to act quickly.

"Syria's history is being written in the blood of her people and it is happening on our watch," said Cameron.

"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," he continued.

Washington and its allies are rapidly approaching an "urgent window of opportunity [in Syria] before the worst fears are realized," Cameron said.

"That's not just true for the United States. That's not just true for Great Britain," Obama added. "But that's also true for Russia."

Obama and Cameron's comments come a week after the U.S. and Russia seemingly reached a breakthrough toward ending the Syrian conflict.

Secretary of State John KerryJohn Forbes KerryFormer Georgia senator and governor Zell Miller dies 2020 Dems compete for top campaign operatives Kentucky candidate takes heat for tweeting he'd like to use congressman for target practice MORE and Russian Foreign Minister Sergei Lavrov agreed to establish a bilateral peace council to set a path for a peaceful end to Assad’s rule and the handover of power to a coalition government. But the council must resolve many contentious issues.

Obama praised the deal on Monday, saying he hoped the move could bring a "peaceful political transition that leads to Assad's departure, but a state in Syria that is still intact.”

“That's not just going to be good for us, that'll be good for everybody," he said.

Washington and Moscow have been at odds over Russia's military support for the Assad regime since the beginning of the Syrian civil war.

Assad's forces have used heavy weapons and air power, supplied by Russia and Iran, to batter opposition forces, often inflicting heavy civilian casualties.

The suspected use of chemical weapons by the Assad regime against rebel forces, crossing a "red line" set by Obama, has further fueled congressional calls for the U.S. to arm the Syrian opposition with small arms and heavy weaponry.

On Monday, Cameron said London is still exploring options to amend a European Union embargo on arms to Syrian rebels.

"We're continuing to examine [the] ... embargo and see whether we need to make further changes to it," Cameron said.

"I do believe that there's more we can do," he added.

But even with full support from Moscow, Obama said he could not guarantee a U.S.-brokered political solution in Syria would hold.

"There are going to be enormous challenges in getting a credible process going, even if Russia is involved," the president said.

"We still have other countries like Iran, and we have non-state actors like Hezbollah that have been actively involved," he added, "On the other side, we've got organizations like [Jabhat] al-Nusra ... that have another agenda beyond just getting rid of Assad."

That "combustible mix" of regional interests and militant groups converging into Syria will pose serious hurdles to the administration's efforts to end the war, according to Obama.

"It's going to be challenging, but it's worth the effort," he added.