US secretary of State John Kerry
US secretary of State John Kerry (Reuters)

US secretary of state John Kerry has given an ultimatum to Syrian president Bashar al-Assad to avoid a military strike by turning over his entire chemical weapons arsenal to the international community within the next week.

At a joint press conference with UK foreign secretary William Hague, Kerry said that America was not going to war but would launch an "unbelievably small and limited effort" to punish the Assad regime for the 21 August chemical weapons attack in Ghouta and to deter it from doing it again.

"If you want to send Assad a congratulatory message, you would support non-intervention," he said although he was sympathetic to fears of the American public who do not want to see troops coming home in body bags.

"But that's not what we are talking about," he stressed. "Military effort will be targeted and short-term."

Kerry added that the only thing Assad's government could do to stop an attack was to turn over all his chemical weapons to the international community within the next week.

"But he is not about to do it and it cannot be done," Kerry added.

He said that the evidence gathered about the chemical attack blaming the Assad regime was "real evidence I could take into a courtroom".

"I've personally tried people who have gone away for long prison sentences or for life for less evidence than we have for this," he said.

"[Assad offers only] words that are contradicted by facts."

Kerry also quoted the late British prime minister Margaret Thatcher in stressing that the UK and America were "real and true friends".

The special relationship between the two countries was as relevant today as it has been in the past, he said.