* Defecting Syrian opposition bloc plans to return
* Seen trying to curb opposition leader's powers
* Opposition divisions reflect Qatar-Saudi rivalry
* Rifts hamper rebel chances of toppling Assad (Adds Jarba's remarks at the Arab League)
By Khaled Yacoub Oweis
AMMAN, March 9 (Reuters) - A large Qatar-backed bloc that left Syria's opposition National Coalition has reversed its decision and wants to rejoin, setting the scene for a clash with the group's Saudi-backed president, opposition sources said on Sunday.
The 40-member bloc, which quit the 120-member coalition before Syrian peace talks began in Geneva in January, said it had returned to confront what it saw as its unfair exclusion from decision-making.
Infighting within the opposition coalition has undermined rebel efforts to take on forces loyal to Syrian President Bashar al-Assad, and has also played into the hands of rival, more hardline Islamist outfits which include foreign militants.
U.S.- and Russian-sponsored talks to end the three-year-old civil war have stalled after two rounds in which the coalition and Assad's representatives failed to make substantive progress.
Addressing Arab foreign ministers at an Arab League meeting in Cairo on Sunday, Jarba said the Geneva talks had suffered a "setback" and called for advanced weapons to be supplied to moderate rebel brigades.
"There is no longer space for diplomatic language or political solutions. They (the government) have met the positive signs on our part by...firing tank guns and dropping barrel bombs on unarmed civilians," he said. "They are people who do not understand anything but force. Only force will deter them."
Bickering in the coalition reflects regional rivalries that pit Qatar against Saudi Arabia and its allies.
Tensions rose last week when Saudi Arabia, the United Arab Emirates and Bahrain withdrew their ambassadors from Qatar in an unprecedented public split between Gulf Arab allies who have fallen out over the role of Islamists in a region in turmoil.
Within the National Coalition, Qatar's candidate for the leadership of the group - former Syrian Prime Minister Riyad Hijab - narrowly lost out in January to the Saudi-backed Jarba.
Jarba faces another leadership vote in July, and coalition sources said the bloc's decision to rejoin the opposition appeared to be aimed at curbing his powers.
"We have decided to resume our political activity in the coalition as a single bloc," the 40 former defectors said in a statement, citing "perilous changes the Syrian revolt is undergoing", an apparent reference to rebel infighting.
The bloc also said it expected the Geneva talks to fail and attacked Jarba's decision to dismiss General Selim Idriss, a commander in the Free Syrian Army (FSA) who opposition sources said had opened channels of communication with Qatar.
Coalition spokesman Monzer Akbik declined to comment on the defectors' decision to rejoin the coalition.
Haitham al-Maleh, a veteran human rights campaigner who heads the coalition's legal committee, said the 40 were entitled to attend the body's next full meeting, which has been delayed but could be held in a few weeks' time in Cairo.
"I hope they will attend," Maleh said. "We are in the midst of a revolution. We are trying to prevent Syria from descending into catastrophe. This is not the time for competition over positions."
But a coalition official, who asked not to be named, warned that the bloc's return could reignite tensions.
"Jarba is now expected to alter the composition of the FSA members to his advantage and strengthen his control of the coalition. The 40 have returned to try to prevent his re-election when it comes up on July 4," the official said.
A pro-Jarba opposition source added: "I think Jarba would be making a mistake if he lets the 40 rejoin. He is being urged by his allies not to allow them back in."
The most influential members of the dissident bloc are Mostafa Sabbagh, a businessman who is Qatar's point man in the coalition, and Hijab, the most senior figure to defect from Assad's government since the revolt began. (Editing by Alistair Lyon)