Kerry issued the ultimatum as he arrived in London on the latest leg of a whistlestop tour of Europe to garner support for intervention in Syria.
Speaking ahead of a crucial vote in Congress on U.S. involvement, he said there was compelling evidence that Assad’s regime had used chemical weapons against its own people.
But in a tacit message to American politicians who have heavily resisted action in the Middle East, Mr Kerry said that any strike would be ‘unbelievably small’.
‘The United States of America – President Obama, myself and others – are in full agreement that the end of the conflict in Syria requires a political solution,’ he said during a joint press conference with Foreign Secretary William Hague.
Congress was in session yesterday for the first time since the August recess. Debate on Syria could begin in the full Senate this week, with voting as early as Wednesday. The House of Representatives could take up the issue later this week or next.
Obama is expected to spend the next several days in personal meetings with members. Some Democratic opponents of a military strike, meanwhile, were looking for a way to spare Obama’s administration the effects of a ‘no’ vote. Obama is scheduled to address the American public on television today, but polls show a majority of Americans opposed to his plan for military action.