The United States said on Thursday it was unable to conclusively say that chemical weapons were used in an alleged deadly gas attack near Damascus, and US President Barack Obama has directed US intelligence agencies to urgently help verify allegations.
"At this time, right now, we are unable to conclusively determine CW (chemical weapons) use," State Department spokeswoman Jen Psaki told reporters. "We are doing everything possible in our power to nail down the facts," she added.
She said Obama had instructed the US intelligence community to look into what happened.
"That means gathering information from witnesses on the ground, it means intelligence gathering, it means open-source reporting, it means scientific gathering," Psaki said, acknowledging that it may be a challenging task given the United States does not have diplomatic relations with Syria.
With the death toll from Wednesday's incident estimated between 500 and 1,300, it would be the world's most lethal chemical weapons attack since the 1980s.
Secretary of State John Kerry has spoken to his counterparts in Europe and the Middle East to better understand what happened and discuss "appropriate steps".
The United States was part of a group of about 36 countries who asked the United Nations on Wednesday to immediately investigate the alleged attacks. Syria's government has offered no immediate public response to calls on Thursday for the UN to have access to the areas.
Obama warned Damascus last year that any attempt to deploy or use chemical or biological weapons would cross a "red line." Psaki on Thursday said that line had been crossed "a couple of months ago" and there were a range of options that Obama and his national security team were considering.
She declined to elaborate on what those actions could be, although the White House said in June it would offer military aid to vetted groups of Syrian rebels who have been fighting for more than two years to oust President Bashar al-Assad.
"If these reports are true it would be an outrageous and flagrant escalation of use of chemical weapons by the regime, so our focus is nailing down the facts," Psaki said.