Obama and Cameron addressed the mediaafter a meeting between the two leaders in Washington on Monday.
"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.
Al Jazeera's Patty Culhane, reporting from Washington DC, said that while there was a lot of talk in local media about the possibility of arming the rebels, there are concerns about which country is arming which Syrian rebel group.
She said it will be a tough battle if the Obama administration decides to arm the rebels.
Cameron said he ruled out tougher action in Syria but pledged to double non-lethal aid to Syria.
Our correspondent said there was no real change in position from the statements made by the two leaders.
She said the Obama administration is under a lot of pressure as some rebels groups arebelieved to belinked to al-Qaeda.
Obamahas also said publicly thatthe use of chemical weapons would be the "red line" that would have to be crossed for theUSto reconsider its position.
Cameron, fresh from a trip to Moscow, one of Assad's few remaining backers, saidthe USefforts that had convincedRussia to join a conference on a political transition in Syria were a significant step forward.
He told National Public Radio that John Kerry, US secretary of state, made a "real breakthrough" in talks with Russian President Vladimir Putin "when they agreed to an American-Russia peace conference".
Cameronalso 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".
Amid the diplomatic developments, reverberations mounted from a string of deadly bombings in the Turkish town of Reyhanli, which theTurkish government blamed on Syria.
Thousands of Turks took to the streets on Sunday to urge their government to rethink its outspoken support for rebelsbattling Assad, warning that the decision had provoked reprisals against Turkey, including the bombings, which killed 48 people.
Recep Tayyip Erdogan, Turkish prime minister,is due to meet Obama at the White House on Thursday, with Syria also topping their agenda.
In another sign of accelerating diplomacy on Syria, the Kremlin said Benjamin Netanyahu, Israeli prime minister, will hold talks on Tuesday with Putin amid concernsRussia plans to deliver advanced missiles to the Assad government.
Arrangements for the peace talks sponsored by Russia and the US, which could take place later this month, meanwhile remain unclear.
Syrian opposition forces said they will consult Saudi Arabia, Qatar and Turkey before deciding whether to take part in the talks.
"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 gave warning on Sunday that the humanitarian aid community was at "breaking point" because of the scale of needs created by the conflict.
Kristalina Georgieva, EU's humanitarian aid commissioner,issued the warning as she visited Syrian refugees in Jordan and unveiled $84min additional aid.
"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," Georgieva said.