The world will not act before having "credible and corroborated facts" about the use of chemical weapons by the Syrian government in its conflict with the opposition forces, the White House said on Thursday.

An unnamed official said more facts are needed to ascertain the intelligence, as the White House sent a letter earlier in the day to Senate leaders saying "The U.S. intelligence community assesses with some degree of varying confidence that the Syrian regime has used chemical weapons on a small scale in Syria, specifically the chemical agent sarin."

"The chain of custody is not clear, so we cannot confirm how the exposure occurred and under what conditions," the official told reporters via a telephone conference.

"If we reach a definitive determination that this red line has been crossed based on credible corroborated information, what we will be doing is consulting closely with our friends and allies and the international community more broadly, and the Syrian opposition, to determine what the best course of action is," he said.

"Given the stakes involved, given how serious the situation is and what we've learned from our own recent experience, intelligence assessments are not alone sufficient," he explained, noting "Only credible and corroborated facts that provide us with some degree of certainty can then guide our decision making."

"All options are on the table," he added.

President Barack Obama has set a red line over chemical weapons, warning time and again that Syrian government's use or failure to secure them will be a "game changer" in his calculation on policy toward the Arab nation.

France and Britain claimed last week that there was evidence of small-scale use of these weapons on several occasions in recent months, an assertion echoed by Israel on Tuesday.

The White House official said Obama was calling on the United Nations to lead a comprehensive evaluation of the evidence and establish what exactly took place.

The United Nations is still involved in technical discussions with Syria about the deployment of an investigation team in the country.

The Obama administration is seeking to oust Syrian President Bashar al-Assad for a political transition in his country, where the conflict has entered its third year and claimed over 70,000 lives. Endit