The Syrian government has used chemical weapons in its conflict with the opposition forces, the White House said on Thursday.

In a letter sent to some members of the Congress, the White House said that "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."

U.S. Defense Secretary Chuck Hagel, who was on a visit to Abu Dhabi, the United Arab Emirates, issued a statement on the latest development saying the conclusion was made in the past 24 hours after assessment for some time.

"We cannot confirm the origin of these weapons, but we do believe that any use of chemical weapons in Syria would very likely have originated with the Assad regime," the secretary said.

"As the letter states, the president has made it clear that the use of chemical weapons or the transfer of such weapons to terrorist groups would be unacceptable," he added.

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

Israel, France and Britain have claimed that the Syrian government headed by President Bashar al-Assad has already used such weapons in its conflict with the rebels, now in its third year with over 70,000 deaths so far.

"The United States has an obligation to fully investigate -- including with all key partners and allies, and through the United Nations -- evidence of chemical weapons use in Syria," Hagel said in his statement.

"As I've said, this is serious business," he added. "we need all the facts."

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