The White House on Thursday again ruled out arming the opposition in Syria after peace envoy Kofi Annan announced his resignation.

"Our position has not changed," White House spokesman Jay Carney told reporters aboard Air Force One en route to Orlando, Florida.

"We provide non-lethal assistance to the opposition. We don't believe that adding to the number of weapons in Syria is what's needed to help bring about a peaceful transition," he said.

He noted that President Barack Obama is "grateful" for Annan's willingness to serve as UN-Arab League joint envoy and for his efforts to achieve peace and a peaceful transition in Syria.

Annan, who assumed his current post about five months ago, told reporters in Geneva on Thursday that he was disappointed over the lack of progress in the Syrian peace process and that he would step down after his current mandate expires on Aug. 31.

The former UN chief said without united international pressure, it is impossible for him and anyone else to compel the Syrian government and the Syrian opposition to take the steps necessary to begin the political process.

"The increasing militarization on the ground and the clear lack of unity in the Security Council have fundamentally changed the circumstances for the effective exercise of my role," Annan said, adding he was frustrated at the "figure-pointing" and "name- calling" in the Security Council.

He criticized the Syrian government for its "continued refusal" of his six-point peace plan and held the opposition responsible for the escalation of the military campaign which has centered on the capital city of Damascus and the northern province of Aleppo for the past days.

Carney claimed that Annan's resignation highlights the continued attacks on the Syrian people by the Syrian government headed by President Bashar al-Assad, despite his promise to abide by Annan's plan.

"The key element here is al-Assad's departure from power," Carney said.

He also criticized Russia and China for their vetoes of draft resolutions on Syria at the Security Council.

As permanent members of the Security Council, Russia and China have advocated a negotiated settlement of the 17-month conflict in Syria, and opposed any foreign interference.