LONDON (AFP) - British Prime Minister David Cameron's office said on Wednesday that he and US President Barack Obama had no doubt Syrian President Bashar al-Assad's regime had used chemical weapons against its own people.

"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".