Barack Obama has called on the US Congress to approve $500m to train and equip vetted Syrian rebels, in what would be a significant escalation of US involvement in the conflict.
The White House on Thursday said it intended to "ramp up US support to the moderate Syrian opposition ... to help defend the Syrian people, stabilise areas under opposition control ... counter terrorist threats, and promote conditions for a negotiated settlement".
The funds would allow the US military to "train and equip appropriately vetted elements of the moderate Syrian armed opposition," said John Kirby, a Pentagon spokesman.
There is no military solution to this crisis and ... the United States should not put American troops into combat in Syria.
The National Security Council's spokeswoman Caitlin Hayden said the funding "would build on the administration's long-standing efforts to empower the moderate Syrian opposition".
Russia's ambassador to the UN, Vitaly Churkin, however said that the US plan was a step in the wrong direction. “I am sure that it is possible to spend it for something more useful,” he said.
The announcement came hours before the Syrian National Coalition, the opposition group afforded diplomatic status by the US, sacked the chief of staff of the Free Syrian Army, its military wing, over corruption allegations.
The SNC said Brigadier General Abdelilah al-Bashir was sacked and his Supreme Military Council disbanded, and called on "revolutionary forces on the ground" to set up a new council within a month and revamp the command structure. It made no further statement on the corruption claims.
Bashir is the second FSA commander to be sacked this year. Salim Idriss lost his job in February after his forces suffered setbacks against other rebel groups, such as the Islamic State of Iraq and the Levant, and forces loyal to the Assad regime.
About $287m in mainly non-lethal support has been cleared for the rebels since March 2011, and the CIA has participated in a secret military training programme in neighbouring Jordan for the 'moderate' opposition.
The $500m in aid is part of a proposed $1.5bn "regional stabilisation initiative" to bolster Syrian neighbours Jordan, Lebanon, Turkey and Iraq, and to support communities hosting refugees.
Hayden said US officials "believe that there is no military solution to this crisis and that the United States should not put American troops into combat."
But she said that the request for the rebels would help Syrians "push back against the growing number of extremists like ISIL, who find safe-haven in the chaos".
Meanwhile the Iraqi prime minister, Nouri al-Maliki, has said he "welcomed" air attacks by Syria against ISIL targets on Iraqi soil, after raids on the border town of al-Qaim on Tuesday.
Maliki told the BBC Arabic that Baghdad did not request the air raids.
"There was no coordination involved, but we welcome this action. We welcome any Syrian strike ... because this group targets both Iraq and Syria ... But we didn't make any request from Syria. They carry out their strikes and we carry out ours. The final winners are our two countries," he said in the BBC interview.