The rebel Free Syrian Army said Sunday it had fired Selim Idriss as its military chief, citing the "difficulties faced by the Syrian revolution" in its battle with the regime.
In a video broadcast on the Internet, the rebel coalition said its military council had decided to replace Idriss with Brigadier General Abdel al-Ilah al-Bachir.
Colonel Qassem Saadeddine said the decision was taken due to "the paralysis within the military command these past months" and the need to "restructure".
A source inside the Syrian opposition told AFP that Idriss — who was appointed to the role in December 2012 — had faced criticism for failings on the battlefield.
These included "errors and carelessness in combat" and "poor distribution of weapons" among the rebels on the ground, the source said.
He is also accused of distancing himself from "the concerns of the insurgents".
The group's new leader Al-Bachir is the head of the military council in the southern province of Quneitra. He deserted the regular army in 2012.
According to the FSA Facebook page, Al-Bachir's son was killed in fighting at the beginning of the year.
Considered the "moderate" rebel group, the Western-backed FSA was once the country's strongest armed opposition force but is now increasingly marginalized by Islamists.
It has been weakened by internal rifts and by competition from other rebel coalitions such as the Islamic Front, a powerful alliance formed last year that is now the largest rebel force with tens of thousands of fighters.
In December the United States and Britain suspended non-lethal aid to the FSA, dealing a major blow to a group that appears caught between advancing regime forces and the increasingly unified Islamists.
The move came after the Islamic Front — which has said it wants to set up an Islamic state in Syria — seized weapons warehouses from the FSA.
The FSA's decision to replace Idriss comes after peace talks between the Syrian regime and the opposition in Geneva ended without reaching any results, throwing the future of the negotiations to end the bloody conflict into doubt.
Britain, France and the United States, which back the uprising against the Syrian regime and have regularly called for President Bashar Assad to stand down, have blamed the Syrian government for sinking the talks.
However Damascus insists the talks did not fail and that "important progress" was made, despite the rivals appearing further apart than ever.
The opposition has insisted that the focus of the talks must be on creating a transitional government, without Assad.
The regime representatives have meanwhile stubbornly insisted that Assad's position is non-negotiable.
The so-called Geneva II process was initiated by the United States, which backs the Syrian opposition coalition, and Moscow, which supports the government.
No date has been set for a third round of talks.
Syria's uprising began as a series of peaceful pro-democracy protests in March 2011, but a brutal regime crackdown ignited a full-blown civil war in which hardline Islamist groups have taken on an increasingly prominent role.
More than 140,000 people have been killed in three years of fighting, according to the Syrian Observatory for Human Rights.