[FORT LEE, VIRGINIA] President Barack Obama said on Wednesday he had yet to hear ideas for ending Syria's civil war that would not involve huge numbers of US troops, even after bringing critics of his policy into the White House Situation Room.
Mr Obama has been under pressure to do more for victims of Syria's five and a half year civil war after the collapse of a US and Russian-brokered truce earlier this month was followed by intense bombing of Aleppo.
Mr Obama told a town hall event at a US military base that the situation was "heartbreaking" and he had re-examined his Syria policy on a near-weekly basis.
"We'll bring in outside experts - I will bring in critics of my policy to find out, 'OK, you don't think this is the right way to go - you tell me what it is you think would allow us to prevent the civil war that's taking place,'" Mr Obama said at the event, televised on CNN.
"In Syria, there is not a scenario in which - absent us deploying large numbers of troops - we can stop a civil war in which both sides are deeply dug in," he said.
Mr Obama said it was important to him to be "judicious" in sending in troops because of the "incredible sacrifices" involved and also because the US military remained involved in Afghanistan and Iraq.
Providing support to Syria's moderate opposition earlier in the conflict would have likely escalated into a conflict with Russia or Iran, which support Syrian President Bashar al Assad, and left the United States "overextended," Mr Obama said.
"There are going to be challenges around the world that happen that don't directly touch on our security, where we need to help, we need to help lead, but just sending in more troops is not going to be the answer," Mr Obama said.