President Barack Obama says he will pursue diplomatic efforts to remove Syria’s chemical weapons but has ordered the US military to “be in a position to respond” if such measures fail.
In a televised address, Barack Obama said he had asked Congress to postpone a vote authorizing the use of force.
The US has threatened air strikes after a chemical weapons attack killed hundreds in Damascus last month.
Russia has proposed such weapons be placed under international control.
Although Syrian officials have agreed in principle, the US and its allies remain skeptical.
The Russian plan triggered a day of diplomatic wrangling at the UN on Tuesday.
Speaking from the White House, President Barack Obama said his administration had long resisted calls for military action in Syria because he did not believe that force could solve the civil war.
But he said he changed his mind after the chemical weapons attack in the Damascus suburbs on August 21.
“The images from this massacre are sickening,” he said.
“On that terrible night, the world saw in gruesome detail the terrible nature of chemical weapons and why the overwhelming majority of humanity has declared them off limits, a crime against humanity and a violation of the laws of war.”
The Syrian government has strongly denied carrying out the attack and instead blamed rebels trying to oust President Bashar al-Assad.
However, Barack Obama said the US “knew” the Assad regime was to blame.
“We know that Assad’s chemical weapons personnel prepared for an attack near an area where they mix sarin gas,” he said.
“They distributed gas masks to their troops. Then they fired rockets from a regime-controlled area into 11 neighborhoods that the regime has been trying to wipe clear of opposition forces.”
Barack Obama said that such an attack was not only a violation of international law it was also a danger to US national security.
“As the ban against these weapons erodes, other tyrants will have no reason to think twice about acquiring poison gas and using them,” he said.
He said that “after careful deliberation” he had decided to respond to the use of chemical weapons through “a targeted military strike”.
“The purpose of this strike would be to deter Assad from using chemical weapons, to degrade his regime’s ability to use them and to make clear to the world that we will not tolerate their use. That’s my judgment as commander in chief.”
However, Barack Obama said he would not “put American boots on the ground in Syria” or pursue open-ended action such as that in Iraq or Afghanistan.
He added: “Others have asked whether it’s worth acting if we don’t take out Assad. As some members of Congress have said, there’s no point in simply doing a pinprick strike in Syria. Let me make something clear: The United States military doesn’t do pinpricks.”
President Barack Obama said he welcomed Russia’s proposal as an alternative to military action, but added: “It’s too early to tell whether this offer will succeed.
“Any agreement must verify that the Assad regime keeps its commitments. But this initiative has the potential to remove the threat of chemical weapons without the use of force.”
Barack Obama said he had therefore asked the leaders of Congress to postpone a vote to authorize the use of force “while we pursue this diplomatic path”.
He confirmed earlier reports that US Secretary of State John Kerry would meet his Russian counterpart Sergei Lavrov in Geneva on Thursday, adding: “I will continue my own discussions with President [Vladimir] Putin.”
“I’ve spoken to the leaders of two of our closest allies, France and the United Kingdom. And we will work together in consultation with Russia and China to put forward a resolution at the UN Security Council requiring Assad to give up his chemical weapons and to ultimately destroy them under international control.”
He added: “Meanwhile, I’ve ordered our military to maintain their current posture, to keep the pressure on Assad and to be in a position to respond if diplomacy fails.”