US Secretary of State John Kerry has kicked off his first official overseas trip by meeting with British leaders in London, as part of a hectic nine-day dash through Europe and the Middle East.

Mr Kerry's maiden diplomatic meeting abroad was with British Prime Minister David Cameron for talks expected to focus on the crisis in Syria and Iran's nuclear programme.

Later, he will see Britain's Foreign Secretary William Hague before flying to Germany for meetings in Berlin.

Mr Kerry arrived in London late last night as the Obama administration launched a frantic effort to try to salvage a Syrian opposition conference this week in Rome.

Some members of the sharply divided Syrian Opposition Council are threatening to boycott the meeting.