US Secretary of State John Kerry, kicking off his first official overseas trip by meeting Monday with British leaders in London, said the “window of diplomacy” for resolving the question of Iran’s nuclear program was still open, but that it would not remain so indefinitely.

Kerry, on the first leg of a hectic nine-country dash through Europe and the Middle East, said the window for a diplomatic resolution, though “open today,” required Iran to negotiate in good faith and by definition “cannot remain open forever.”

Kerry’s first diplomatic meeting abroad came with British Prime Minister David Cameron, where they discussed the prospects of a free trade agreement between the United States and the European Union and the importance of the Middle East peace process.

Cameron’s spokesman, Jean-Christophe Gray, said the prime minister and Kerry also “reiterated their shared determination to prevent a nuclear-armed Iran” ahead of talks between world leaders and Tehran in Almaty, Kazakhstan, this week.

The two also spoke about the challenges posed by fragile states such as Syria, Libya and Egypt, the spokesman added.

Later, Kerry was expected to discuss the Falkland Islands with UK Foreign Secretary William Hague before flying to Germany for meetings in Berlin.

Kerry arrived on London late Sunday as the Obama administration launched a frantic effort to try to salvage a Syrian opposition conference that Kerry plans to attend this week in Rome. Some members of the sharply divided Syrian Opposition Council are threatening to boycott the meeting.