Both sides said the Tuesday meeting was productive and marked an important step in the evolving relationship between the United States and the opposition.

It took place however as the Obama administration again voiced concerns that any deadly aid that was provided at the request of militants in Syria could end up in the hands of extremists.

Obama dropped by a meeting between Jarba, the head of the so-called Syrian National Coalition (SNC), and his national security advisor Susan Rice.

Jarba, according to a White House statement, thanked Obama for a total of $287 million in US aid to opposition forces and noted the US role as the largest humanitarian donor to Syrian refugees with a total aid grant of $1.7 billion.

But there was no mention in the statement of Jarba's previous plea to the administration for anti-aircraft weapons.

US officials privately acknowledged he made the request in talks with Secretary of State John Kerry at the State Department last week, but they refused to be drawn on the response.

Washington is worried that such weapons could eventually end up in the hands of groups hostile to the United States or its allies and could even pose a threat to commercial aircraft.

Jarba's visit came at an inauspicious time for opponents of President Bashar al-Assad, after terrorist groups pulled out of the battleground city of Homs and UN and Arab League mediator Lakhdar Brahimi resigned after failing to broker a political solution to the war which has claimed 150,000 lives.

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