The United Nations and the United States are calling for an immediate investigation of Syrian activists' claims that the Bashar al-Assad government used chemical weapons in an attack on civilians.
Anti-regime activist groups in Syria say more than 1,300 people were killed in the attack outside Damascus, many of them women and children. Video footage and witness reports appeared to bolster claims that chemical weapons were used.
President Barack Obama has directed the U.S. intelligence community to urgently gather additional information to try to assess whether chemical weapons were used Wednesday, State Department spokeswoman Jen Psaki said Thursday.
At this time, she said, the United States is unable to "conclusively determine" chemical weapons use, but is focused on trying to nail down the facts, along with its international partners.
Psaki said, as she has before, that if reports of chemical weapons use prove true, the president has a range of options available to him to respond.
Later, a senior defense official told CNN that "the military continues to refine options for Syria to be prepared for whatever the president might request down the line."
Psaki noted that Secretary of State John Kerry spoke by phone Thursday to Syrian opposition leader Ahmad Jarba; U.N. Secretary-General Ban Ki-Moon; and French, Jordanian, Turkish and European Union leaders.
Ban Ki-moon said the alleged incidents Wednesday need to be probed "without delay."
Ban urged a Damascus-based team "to be granted permission and access to swiftly investigate the incident" and the United Nations is sending a formal request to Syria.
"He expects to receive a positive response without delay," his spokesperson said in a statement.
U.N. High Commissioner for Human Rights Navi Pillay said the matter is "of utmost urgency" and the "allegations are exceptionally grave."
Pillay urged both the government and opposition to enable investigators "to examine the site of the alleged attacks without any delay or obfuscation."
"The use of chemical weapons is prohibited under customary international law," she said, noting that the prohibition is binding on the government and rebels.
"Whether or not chemical weapons were in fact used, it seems that once again in Syria many civilians have been killed in flagrant contravention of international law."
U.N. special advisers Adama Dieng and Jennifer Welsh similarly urged immediate access for the U.N. investigation.
"There is never any military justification for the use of chemical weapons -- whether by governments or anti-government armed groups -- given their horrific and indiscriminate impact," they said in a joint statement.
The reports prompted international outrage, with a U.N. Security Council briefing called late Wednesday to discuss the situation. However, Russia and China -- consistent allies of the Syrian government -- reportedly blocked a formal resolution.
Hours after the closed-door meeting, French Foreign Minister Laurent Fabius told CNN affiliate BFMTV that "a reaction of force must be taken" if the allegations are true.
"If the U.N. Security Council cannot do it, decisions will be made otherwise," Fabius said. But, he said, sending ground troops to Syria is out of question.