U.S. Secretary of State John Kerry arriving in Qatar on Saturday to attend a summit of the so-called 'Friends of Syria'.
The summit, which brings together foreign ministers of 11 pro-rebel countries, comes in the wake of an American decision to arm some elements of the opposition.
The move has brought a chill to relations between Washington and Moscow.
Russian President Vladimir Putin warned against arming the opposition which he said included "terrorist" groups, but defended his own country's arms sale to the Syria regime.
The rebels say they urgently need advanced weaponry, including anti-tank and anti-aircraft weapons, to stem the recent advances made by forces loyal to President Assad.
But there are concerns in Western nations that such advanced weaponry could fall into the hands of jihadi groups hostile to the West.
A UN human rights investigator also warned on Friday, that an increased flow of arms to both sides would likely result in further w