Cameron: "Syria's history is being written in the blood of her people and it's happening on our watch."
During a meeting in Washington on Monday, US President Barack Obama and British Prime Minister David Cameron reportedly agreed to "increase the pressure" on - and aim for the departure of - Syrian President Bashar al-Assad.
"We're going to continue our efforts to increase pressure on the Assad regime, to provide humanitarian aid ... to strengthen the moderate opposition and to prepare for a democratic Syria without Bashar al-Assad," Obama said.
According to Al Jazeera's reporter Patty Culhane, the discussions surrounding a potential arming of the rebels are marred with questions around which Syrian rebel groups are already provided weapons and by whom.
The Obama administration is therefore facing a cleaving decision whether or not to arm the rebels, especially as some of them are believed to be connected to al-Qaeda, Patty Culhane reported.
Cameron dismissed any potential implication in the Syria conflict but pledged to double non-lethal aid to Syria.
Obama has also said publicly that the use of chemical weapons would be the "red line" that would make the US to reconsider its position over non-intervention in Syria.
Cameron hailed the US efforts to secure Russia’s participation in a conference on a political transition in Syria as a great progress.
Russia is the last country supporting Assad’s regime and has been one of the main obstacles in finding a solution to end the Syrian crisis.
He also said that Putin was "keen now to move from the generalities of having a peace conference to talking through the specifics of how we can make [this] work.
"There are still big hurdles to overcome ... but I sense there is an understanding now that the current trajectory of Syria ... this is not in anybody's interest".
Syrian opposition forces’ participation in the talks was still to be confirmed.
"It is too early to decide whether or not we will take part, because the circumstances of this conference are not yet clear," George Sabra, acting head of the opposition National Coalition, said in Istanbul.
"There is no agenda or calendar yet. The list of participating states and their representatives has not yet been announced."
The European Union, after pledging an $84 million additional aid, also warned on Sunday that the needs created by the two-year long conflict required increased humanitarian relief support.
"Unless all those involved in the fighting, as well as the international community, find a political solution to the violence very soon, the humanitarian community will simply be unable to cope with the unprecedented scale of the needs - we are already at breaking point," Kristalina Georgieva, EU's humanitarian aid commissioner, said.