President Barack Obama said on Saturday he was willing to give diplomacy a chance to help resolve the Syrian crisis, but warned the military option was still on the table.
US Secretary of State John Kerry and his Russian counterpart Sergei Lavrov were engaged in a third day of talks following Moscow's surprise initiative to finalize an agreement on eliminating Syrian President Bashar al-Assad's chemical weapons.
"We are not just going to take Russia and Assad's word for it. We need to see concrete actions to demonstrate that Assad is serious about giving up his chemical weapons," Obama said in his weekly address.
"And since this plan emerged only with a credible threat of US military action, we will maintain our military posture in the region to keep the pressure on the Assad regime."
The Russian plan has led Obama to put on hold planned military strikes in response to an August 21 chemical attack outside Damascus that Washington blames on the regime and says killed more than 1,400 people.
"We're making it clear that this can't be a stalling tactic," Obama said of the discussions in Geneva.
In Geneva, Kerry said the United States and Russia had clinched a deal that would include the threat of force and under which Syria must present details of its chemical weapons stockpile within a week.
"Any agreement needs to verify that the Assad regime and Russia are keeping their commitments: that means working to turn Syria's chemical weapons over to international control and ultimately destroying them," Obama said.
Signup to our newsletter and follow us on Facebook and Twitter!
"This would allow us to achieve our goal -- deterring the Syrian regime from using chemical weapons, degrading their ability to use them, and making it clear to the world that we won't tolerate their use."
The United States has estimated that Syria possesses around 1,000 metric tonnes of various chemical agents, including mustard and sarin gas, sulfur and VX.
The Russian estimates had been initially much lower, a senior US administration official said.
Obama pointed to "indications of progress" after the Assad regime acknowledged for the first time that it possessed chemical weapons and applied to join the 1993 Chemical Weapons Convention.
"We'll keep working with the international community to see that Assad gives up his chemical weapons so that they can be destroyed," the president said.
"We will continue rallying support from allies around the world who agree on the need for action to deter the use of chemical weapons in Syria.
"And if current discussions produce a serious plan, I'm prepared to move forward with it."
Invoking a "duty to preserve a world free from the fear of chemical weapons for our children," Obama repeated his call for an international response.
"A dictator must not be allowed to gas children in their beds with impunity. And we cannot risk poison gas becoming the new weapon of choice for tyrants and terrorists the world over," he said.
"But if there is any chance of achieving that goal without resorting to force, then I believe we have a responsibility to pursue that path."