US President Barack Obama has met Syrian opposition leader Ahmad Jarba in a show of support for the embattled foes of Syrian President Bashar al-Assad.

The meeting on Tuesday came as the US again voiced concerns that any deadly weapons provided at the request of rebels in Syria could end up in the hands of armed groups it considers extremists.

Jarba, president of the Syrian National Coalition (SNC), also met Obama's national security adviser Susan Rice.

The White House said it condemned the Assad government's "deliberate targeting of Syrian civilians" with barrel bombs and the denial of humanitarian help in areas under siege by the Syrian army.

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A statement by the White House said that Jarba thanked the US for its aid to opposition rebels and its humanitarian help to Syrian refugees, which totals $1.7bn.

There was no mention in the statement of Jarba's previous request to the US for anti-aircraft weapons to combat bombing raids by Assad's forces, the AFP news agency reported.

Officials privately acknowledged that he made the request in talks with Secretary of State John Kerry last week, but they refused to tell reporters Kerry's response.

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

Jarba's visit came on the same day the UN mediator, Lakhdar Brahimi, announced plans to step down from his position on May 31 after failing to broker a political solution to the conflict that has killed 150,000 people so far.

Obama has been reluctant to commit American forces to another foreign conflict, after its wars in Afghanistan and Iraq.

Source: Agencies