British Foreign Office said Thursday they had "limited but persuasive information" showing chemical weapons had been used in Syria.

"We have limited but persuasive information from various sources showing chemical weapon use in Syria, including sarin," a Foreign Office spokesman said in a statement.

However, the spokesman did not specify which forces in Syria allegedly used chemical weapons.

"This is extremely concerning. Use of chemical weapons is a war crime. We have briefed our allies, partners and the UN on this information and we are working actively to get more and better information," the spokesman said.

"Assad must cooperate with the international community and prove that his regime has not committed this horrific crime, allowing unrestricted access for the UN and OPCW (Organization for the Prohibition of Chemical Weapons) to investigate on the ground in Syria," he added.

Earlier on Thursday, the White House said in a letter sent to some members of the Congress, "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 said 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," Hagel said.

Earlier this month, Syrian information minister dismissed as "illegal" a recent Western report, which cited a smuggled-out soil sample from Syria as proving the use of chemical weapons.

"Any testing process to the Syrian soil, which is not performed by official and international organizations with the consent of the Syrian government, is stripped off any political and legal value," Omran al-Zoubi said, adding that the report is "mere forgery against Syria."

Meanwhile, the minister singled out Britain, Turkey and France, saying that those countries know the source of the chemical weapons, referring to their implication in rendering arms to the rebels on the ground.

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