Britain's Prime Minister David Cameron and US President Barack Obama. (AFP)
"Both leaders agreed that all the information available confirmed a chemical weapons attack had taken place, noting that even the Iranian President and Syrian regime had conceded this," Downing Street said after the two leaders spoke on the phone on Tuesday night.
"They both agreed they were in no doubt that the Assad regime was responsible," it added in a statement.
The prime minister's office said the evidence clearly indicated Assad's forces had carried out a chemical weapons attack in the Damascus suburbs last week thought to have killed hundreds of people.
"Regime forces were carrying out a military operation to regain that area from the opposition at the time; and there is no evidence that the opposition has the capability to deliver such a chemical weapons attack," it said.
"The PM confirmed that the government had not yet taken a decision on the specific nature of our response, but that it would be legal and specific to the chemical weapons attack."
The statement came as Cameron prepared to chair a meeting of Britain's National Security Council later on Wednesday.
The prime minister's office calls his talks with Obama "an opportunity for the PM to hear the latest US thinking on the issue and to set out the options being considered by the government".
Also on Wednesday, a team of United Nations inspectors left their Damascus hotel and headed to the scene of an alleged chemical weapons strike, a Reuters witness said.
'Volcano of Revenge' invasion
Meanwhile, an al-Qaeda affiliate threatened a "Volcano of Revenge" invasion against Syrian government security and military targets in retaliation for a suspected poison attack near Damascus, the SITE Monitoring Group reported late on Tuesday.
A branch of the Islamic State in Iraq and the Levant said in a statement it would punish Syria for a series of massacres, including last week's alleged chemical weapons attack, after meeting eight Syrian factions.
Opponents of Assad said his forces used rockets loaded with poison gas in the middle of a fierce offensive on the Damascus outskirts, with activists putting the death toll between 500 and more than 1 000. Assad denies the charges.
"The meeting factions decided to carry out the 'Volcano of Revenge' invasion in response to the regime's massacres against our people in Eastern Ghouta, the last of which was the chemical weapons massacre," SITE quoted the statement, dated August 26, as saying.
"They have decided to strike the main joints of the regime in imprisoned Damascus, including security branches, support and supply points, training centres, and infrastructure," it said.
Syria's government denies any role in the attack and blames rebels. – Reuters, AFP