Kerry is attending a lunch alongside the Syrian opposition's interim prime minister and other senior figures. He later meets one-on-one with Russian Foreign Minister Sergey Lavrov.
Kerry is in London for a G8 foreign ministers' meeting Wednesday and Thursday. He arrived from the Middle East, where he held three days of talks with Israeli and Palestinian leaders.
Before leaving Israel, he said the Obama administration was holding intense talks on how to boost aid to Syria's rebels. He said it was important to increase pressure on Assad's regime to get it to the negotiating table.
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