U.S. President Barack Obama has promised a "vigorous investigation" into reports that Syrian forces have used chemical weapons.
Speaking in Washington on April 26, Obama also renewed his warning that proof of chemical weapons use would be a "game changer."
Obama delivered the warning during talks at the White House with King Abdullah II of Jordan. Obama told reporters that U.S. authorities had "some evidence that chemical weapons have been used on the population in Syria, these are preliminary assessments, they're based on our intelligence gathering."
Obama has previously said chemical weapons use would cross a "red line" and prompt a major U.S. response.
Also on April 26, White House spokesman Jay Carney maintained that reports about Syrian President Bashar al-Assad's regime using chemical weapons were not "airtight" and more time was needed to establish the facts.
"We are working to establish credible and corroborated facts to build on this intelligence assessment in order to establish a definitive judgment as to whether or not the president's red line has been crossed and to inform our decision making about next steps," he said.
Carney also indicated that there was no timeline for further action by the United States.
"Now that we have a growing body of evidence that suggests that we can say with varying degrees of confidence that [Syrian President Bashar al-Assad] has, in a limited way, or that the regime has, in a limited way, likely used [chemical] weapons, we need to build on that," he said. "It is absolutely the correct thing to do to take the exceptional work that our intelligence community does and continue to build information."
The statements from Washington came as the European Union asked Damascus on April 26 to allow a United Nations chemical weapons probe in Syria.
The request from the EU comes after British Prime Minister David Cameron earlier said that there is "limited but growing" evidence that Damascus is using chemical weapons.
Cameron told the BBC on April 26 that the evidence is "very disturbing" and would amount to a war crime if proven.
There has been no official reaction from Damascus.
On April 25, the White House announced that U.S. intelligence agencies have concluded "with varying degrees of confidence" that Assad's regime had used chemical weapons, including the use of the nerve gas sarin.
With reporting by Reuters and AFP