General Salim Idriss has rejected his dismissal as military chief of the rebel Free Syrian Army, saying the whole chain of command needed a "total restructuring" - highlighting deep divisions in the opposition.

Speaking in a video statement on Wednesday, flanked by several top field commanders of the FSA's Supreme Military Council, the sacked rebel chief said: "We ... have been asked to start working on a total restructuring of the SMC."

Idriss lashed out at the opposition Syrian National Coalition defence minister, Assaad Mustafa, who reportedly backed his replacement on Sunday by Brigadier General Abdel Ilah al-Bashir.

He described Mustafa's decisions as "improvised and individual"

With the alleged backing of Mustafa and opposition chief Ahmad Jarba, the FSA's larger Higher Military Council had on Sunday replaced Idriss, citing the "difficulties faced by the Syrian revolution" in its battle with the regime. But several rebel leaders criticised the move, with some branding it an undemocratic "coup".

"We consider the removal of... Idriss an invalid, illegitimate decision," said a statement issued by all five top field commanders of the SMC, which Idriss had led from December 2012.

They vowed to continue fighting President Bashar al-Assad's regime "under the leadership" of Idriss who was "elected democratically".

Idriss had been voted in by military councils on the ground. A rebel source on Wednesday told the AFP news agency that Idriss's removal was decided in a "secret meeting" of the Higher Military Council, which many key rebels have abandoned in recent months.

"Regardless of Idriss's shortcomings, this is a military coup," said the rebel.

"The main problem is: why weren't all the military councils called in to vote?"

Idriss had long faced criticism by rebels on the ground for failing to secure more weapons from foreign backers.

"The Supreme Military Council has in the past year received only $3 million in assistance," the source added.

Britain and the US suspended their non-lethal aid to Idriss's FSA in December after armed fighters seized its warehouses on the Syrian-Turkish border.

But activists on the ground scoffed at the uproar in the SMC over Idriss's dismissal, describing the body as "irrelevant".

"The SMC is very weak and represents nothing in comparison with the big groups fighting on the ground," said Nazeer al-Khatib from Aleppo.