At the conclusion of the G20 summit in St Petersburg, Mr Obama said he had been "encouraged" by his talks with foreign leaders but that the humanitarian situation in Syria and along its borders was "only getting worse".
Mr Obama and Russian president Vladimir Putin have stuck to their positions over Syria, which has dominated talks at the summit.
Addressing reporters, Mr Putin claimed the chemical attack that killed 1,429 people in Damascus last month was a provocation by rebels to encourage military intervention, disputing US claims that the Syrian government was responsible.
He also suggested that a punitive US military strike on the Middle Eastern nation would harm the global economy and stifle growth.
Mr Obama held a "candid and constructive conversation" with Mr Putin for 20 minutes on the sidelines of the summit on Friday but the pair remain at odds.
The Russian president told reporters: "We spoke sitting down ... it was a constructive, meaningful, cordial conversation. Each of us kept with our own opinion."
Mr Obama said he will address the US about Syria on Tuesday as he seeks Congress' authority for action. He refused to say what he would do if the strikes are not approved.Meanwhile, 11 countries - namely Australia, Canada, France, Italy, Japan, Korea, Saudi Arabia, Spain, Turkey, the UK and the US - have released a joint statement condemning "in the strongest terms" the August 21 attack.
"The international norm against the use of chemical weapons is longstanding and universal," it said.
"The use of chemical weapons anywhere diminishes the security of people everywhere. Left unchallenged, it increases the risk of further use and proliferation of these weapons."
The countries said they would support a UN resolution but said the Security Council was "paralysed", adding: "The world cannot wait for endless failed processes that can only lead to increased suffering in Syria and regional instability."
However, there is strong international resistance to military action in Syria.
China is firmly against an attack on Bashar al Assad's regime and the European Union is sceptical about whether the use of force can be effective.
Even Pope Francis has weighed in, urging leaders to abandon what he called a "futile mission".
The US has ordered the evacuation of all non-essential embassy staff and other American citizens in Lebanon due to security concerns as the debate over military action continues.
"The Department of State drew down non-emergency personnel and family members from Embassy Beirut due to threats to US Mission facilities and personnel," a statement on the Beirut embassy's website said.
It also reducing its diplomatic presence in Adana in southern Turkey.
In his closing speech David Cameron said the international community cannot "contract out" its morality by allowing Russia to block intervention in crises such as that engulfing Syria.
The Prime Minister insisted military action had to be possible without an endorsement from the UN Security Council.
Mr Cameron said: "If we live in a world where the US president draws a red line and says that consequences must follow, and no consequences do follow, I fear for that world because not just the dictator in question - Assad - will, I believe, think he has the impunity to use chemical weapons further but other dictators, other countries, will draw the same lesson."
Earlier Syria's state news agency SANA said the country's head of parliament has urged the US Congress to vote against military action targeting its regime.
"We urge you not to take reckless measures as you have the power to steer the United States from the path of war to that of diplomacy," it quoted parliament chief Jihad al Lahham.
Mr Cameron has announced an additional £52m in aid to help the estimated two million refugees who have fled Syria's civil war.
The money will be go towards providing both internally displaced people and refugees in neighbouring countries like Jordan, Lebanon and Iraq with food, water and shelter.
