President Obama said today he had to be sure of the facts about Syria's use of chemical weapons on its own people, before taking what he called "additional action".
He did say options had been prepared but it would take time to decide whether a red line had been crossed by Syia.
ITV News Washington Correspondent Robert Moore reports:
President Obama has again warned that the use of chemical weapons in Syria would be a “game changer”.
He said the US now had evidence that chemical weapons were used inside of Syria, but those responsible had not yet been identified.
He said the US needed to establish the facts before taking additional action.
He added that the Pentagon has been asked for range of options if it was proved that Bashar Al-Assad had used chemical weapons.
Barack Obama has emphasised US concerns about Syrian chemical weapons in a phone conversation with Russian President Vladimir Putin today, the White House said.
"President Obama and President Putin reviewed the situation in Syria, with President Obama underscoring concern over Syrian chemical weapons," a White House statement said.
The two leaders agreed to stay in touch on the issue and that Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov would continue discussions on Syria.
United Nations Secretary-General Ban Ki-moon has renewed his request for complete access to Syria for the UN team investigating the alleged use of chemical weapons in the country.
Speaking in New York, he said: “A credible and comprehensive inquiry requires full access to the sites where chemical weapons are alleged to have been used. I again urge the Syrian authorities to allow the investigation to proceed without delay and without any conditions,”
The initial focus of the investigation will be the alleged use of chemical weapons in Kfar Dael region in Aleppo governorate.
A "line has been crossed", President Obama told reporters at the White House today, after reports emerged that chemical weapons were being used in Syria.
Mr Obama said the use of chemical weapons and the "dangers that it poses" to the international community "added urgency" to the situation.
There needs to be a "political transition" in Syria, President Obama said today, after reports emerged that chemical weapons were being used in the country.
Mr Obama said that the US will be working with countries such as Jordan to get confirmation on use of chemical weapons in Syria, during Jordan's King Abdullah's visit to the country.
Mr Obama said if Syria was to use potential weapons of mass destruction on civilians, "that would cross another line".
"That would be a game changer," he added.
The "limited but growing evidence" that chemical weapons have been used is Syria's civil war must spur the international community into doing more, the Prime Minister said today.
David Cameron described United States intelligence reports pointing to their use as "extremely serious" and said he had "always been keen for us to do more".
ITV News Middle East Correspondent John Ray's report, contains distressing images:
The White House has said it will not set a timeline on how long it will take the US and others to corroborate evidence on Syria's reported chemical weapon use, according to Reuters.
"I'm not going to set a timeline, because the facts need to be what drives this investigation, not a deadline," White House spokesman Jay Carney told reporters at a briefing
"We are continuing to work to build on the assessments made by the intelligence community, that the degrees of confidence here are varying, that this is not an airtight case," he added.