US Secretary of State John Kerry on Tuesday said he would meet representatives of the Syrian opposition in London, speaking shortly before taking off from Israel for the British capital.
"I will be meeting with the Syrian opposition in London," he told reporters before departing from Ben Gurion airport near Tel Aviv after three days of talks with Israeli and Palestinian leaders.
But he said that Syrian rebel leader Ahmed Moaz al-Khatib was not expected to attend the meeting which would be hosted by British Foreign Secretary William Hague.
"We're working on a list of attendees," he said.
Ahead of his arrival in Israel, Kerry was in Turkey on Sunday where he discussed Syria's two-year civil war, Turkish-Israeli rapprochement and the Middle East peace process.
The conflict in Syria, which began as a peaceful uprising against President Bashar al-Assad in March 2011, has repeatedly threatened to spill over into neighbouring countries, including Turkey and Israel.
So far, at least 70,000 people have been killed, UN estimates say.
The US "preference... is to have a diplomatic solution" to the conflict and "a legitimate transfer of governing responsibility to an independent entity," Kerry said.
"It sounds good, (but) the problem is you can't get there if President Assad is unwilling to decide that he should transfer that authority," he added.
Washington has given hundreds of millions of dollars in humanitarian aid but has stopped short of providing lethal military help.
Asked if the US might step up help to Syrian rebels including military aid, Kerry said it was "up to the White House to make any announcement."
At an Arab League summit in Doha last month, Khatib said he had asked Kerry to provide Patriot missile protection that encompasses northern Syria, and that Washington's top diplomat had "promised to look into the matter."
Syria's opposition umbrella group the National Coalition is recognised by the United States and many other Western and Arab countries as the sole representative of the Syrian people.
The Syrian opposition was formally granted an Arab League seat in March.