United Nations inspectors say there is "clear and convincing evidence" that chemical weapons were used on a relatively large scale in an attack last month in Syria that killed hundreds of people.
News agencies are reporting the inspectors said collected environmental, chemical and medical samples show "that surface-to-surface rockets containing the nerve agent sarin were used in the Ghouta area of Damascus'' on August 21.
The inspectors were charged with determining if chemical weapons were used and not who unleashed them. The U.S. says Syrian forces were behind the attack which Washington says killed more than 1,400 last month.
Syria says it was rebel fighters, not government forces, who are responsible.
On the first page of their report to Secretary-General Ban Ki-Moon, the full version of which will be issued later Monday, the inspectors concluded that "chemical weapons have been used in the ongoing [Syrian] conflict against civilians including children on a relatively large scale."
Earlier Monday, the U.S., France and Britain agreed on the need for a strong United Nations resolution that sets precise and binding deadlines for Syria to give up its chemical weapons.
British Foreign Secretary William Hague said Monday the task is difficult and complex, but stressed the need to maintain pressure on President Bashar al-Assad's government.
"It is the Assad regime that has stockpiled these weapons and that has used them repeatedly against the Syrian people, so the pressure is on them to comply with this agreement in full. The world must be prepared to hold them to account if they don't, and our three countries are certainly determined to do so."
Hague spoke alongside French Foreign Minister Laurent Fabius and U.S. Secretary of State John Kerry after they met in Paris. The talks came days after the U.S. and Russia announced a plan that calls on Syria to detail its chemical weapons stockpile. The Syrian government has promised to comply.
Kerry said Monday that anything less than full compliance by the Syrian government with a U.N. resolution would not be accepted. He also expressed the need to find a lasting solution beyond taking chemical weapons away from Mr. Assad's forces.
"We understand that removing the chemical weapons still leaves him with artillery and airplanes and he uses them indiscriminately against his people, and we are going to do everything in our power to continue to work towards the political resolution that is so critical to ending that violence."
Earlier efforts to address the Syrian crisis at the U.N. Security Council have failed, with Russia and China using their veto power to block any outside intervention.
Later Monday in New York, U.N. Secretary-General Ban Ki-moon will brief the Security Council and the General Assembly on the report.