WASHINGTON- The United States now has little doubt the Syrian government used chemical weapons against civilians last week, and President Barack Obama is studying how to respond, a senior official in the US administration said on Sunday."Based on the reported number of victims, reported symptoms of those who were killed or injured, witness accounts, and other facts gathered by open sources, the US intelligence community, and international partners, there is very little doubt at this point that a chemical weapon was used by the Syrian regime against civilians in this incident," the official told Reuters.Syria says granted access to UN to chemical weapons siteSyria has agreed to allow UN inspectors access to sites in the suburbs of Damascus where alleged chemical attacks occurred last week, the Syria Foreign Ministry said in a statement broadcast on state television."The Syrian government and the United Nations agreed on a common understanding ... to allow the United Nations to investigate allegations of chemical weapons use in the Damascus suburbs on Aug 22, 2013," the statement said, giving the wrong date for the mass poisoning, which took place on Aug 21.Many hundreds of people were poisoned to death on Wednesday before dawn in what appears to have been the world's worst chemical weapons attack since Saddam Hussein's forces gassed thousands of Iraqi Kurdish villagers in 1988.Syria has denied that it was to blame for last week's incident. Rebels and many Western officials believe the poisoning was caused by a chemical agent used in a rocket attack carried out by government forces.The incident took place just three days after a UN chemical weapons team arrived in Syria to investigate other smaller allegations of poison gas use. The inspector team's movements must be agreed with the Syrian authorities.The Syrian statement said that the date and time of the inspectors' visit to the site had been agreed, but it did not say when it would take place.Syrian Foreign Minister Walid al-Moualem met UN High Representative for Disarmament Affairs Angela Kane - who was in Damascus to negotiate access - on Sunday morning, it said.Moualem "stressed Syria's readiness to cooperate with a team of investigators to uncover false allegations by terrorist groups that Syrian troops used chemical weapons in (Damascus).""We are continuing to assess the facts so the president can make an informed decision about how to respond to this indiscriminate use of chemical weapons," the official added.