The head of the Syrian opposition coalition's military branch has refused to recognize the body's new prime minister, saying he was not properly elected.

Louay Almokdad, aide to Gen. Salim Idris, told The Associated Press by phone Sunday that the rebel Free Syrian Army would not recognize the U.S.-educated IT expert chosen last week to head a rebel interim government.

Almokdad said Ghassan Hitto was elected without the support of prominent Syrian opposition figures and that a recognized prime minister must have broad support.

Hitto received 35 out of 48 votes cast by the 63 active members of the opposition Syrian National Coalition last week.

Many prominent opposition figures boycotted the vote, saying Hitto was unknown and backed by outside powers like Qatar and the Muslim Brotherhood.

THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.

The leader of the Western-backed Syrian opposition coalition resigned Sunday, citing what he called the lack of international support for those seeking to topple President Bashar Assad.

The resignation of Mouaz al-Khatib deals a blow to the most credible body seeking to represent the opposition, which remains deeply divided and continues to struggle to present a united front two years into Syria's bloody uprising.

Al-Khatib, a respected preacher who has led the Syrian National Coalition since its creation late last year, said in a statement posted on his Facebook page that he was making good on a vow to quit if certain undefined "red lines" were crossed.

"I am keeping my promise today and announcing my resignation from the National Coalition so that I can work with freedom that is not available inside the official institutions," he said.

He also blamed world powers for providing insufficient support for the rebel cause and complained that many "international and regional parties" insisted on pushing the opposition toward dialogue with the regime. Most opposition leaders and activists say Assad's regime has killed too many people to be part of a solution to the conflict.

"All that has happened to the Syrian people _ from destruction of infrastructure to the arrest of tens of thousands to the displacement of hundreds of thousands to other tragedies _ is not enough for an international decision to allow the Syrian people to defend themselves," the statement said.

Al-Khatib was chosen to serve as president of the Coalition, which was formed in November under international pressure to serve as the opposition's official liaison with other countries and coordinate anti-Assad forces inside and outside of Syria.

Despite electing a new, U.S.-educated prime minister to head a planned interim government last week, the Coalition has failed to establish itself as the top rebel authority on the ground in Syria, where hundreds of independent rebel brigades are fighting a civil war against Assad's forces.

The Coalition did not immediately respond to al-Khatib's resignation.

Al-Khatib's spokesman, Ali Mohammed Ali, confirmed the authenticity of the statement in a phone call with The Associated Press. He declined to discuss any issues inside the Coalition that could have influenced al-Khatib's decision.

Speaking on Al Arabiya TV, the former head of the Syrian National Council, which preceded the coalition, said that he and other coalition members were surprised by the resignation.

Burhan Ghalioun also said he assumed the resignation was a protest against world powers that have not provided the opposition with the aid it needs, unnamed countries that have interfered in the coalition's work and other coalition members who have impeded al-Khatib's work.

"I lived this, so I know what it means," Ghalioun said, speaking of his own resignation as head of the SNC last year.

Observers and some members of the Coalition have complained that Qatar, which heavily finances the opposition, and the Muslim Brotherhood exercise outsized power inside the Coalition.

Secretary of State John Kerry said he was sorry to learn of al-Khatib's resignation, but that it won't affect U.S. cooperation with the Coalition on aid.

He called such transitions natural, adding that it shows "an opposition that is bigger than one person and that opposition will continue."

The Syrian government has largely ignored the opposition and says the civil war is an international conspiracy to weaken Syria.

Syria's conflict has split regional and world powers, with some backing the rebels and others standing by Assad. Russia, China and Iran remain the regime's strongest supporters.

On Sunday, Kerry told reporters during an unannounced trip to Baghdad, that he had made it clear to Iraq, Syria's eastern neighbor, that it should not allow Iran to use its airspace to shuttle weapons and fighters to Syria.

Kerry said he told Iraqi Prime Minister Nouri al-Maliki that the transfer of anything that supports President Bashar Assad and his regime is "problematic."

Also Sunday, rebel fighters inside Syria pressed ahead with their offensive in a restive southern province that borders Jordan, as Israel's military said its forces responded to fire by shooting at a target inside Syria.

A victory on the frontier with Jordan would be a significant advance for the opposition. It would deprive Assad of control over a supply lifeline also used by refugees fleeing his military onslaught, and could facilitate the entry of arms and equipment to the rebels.

Since summer, 2012, rebels have seized control of large swathes of land near the Turkish and Iraqi borders to the north and east, respectively, and used these areas to organize their forces and build supply lines. But the opposition has struggled so far to carve out a similar area in the south from which they could organize and marshal their forces for a more sustained push north toward Damascus.

Rami Abdul-Rahman of the Britain-based Syrian Observatory for Human Rights said heavy clashes raged in three towns in the southern Daraa provice on Sunday.

"The rebels are trying to take over more army checkpoints and installations in Daraa," he told the Associated Press, reporting fighting in at least three towns.

A Jordanian border official said he heard heavy artillery and saw smoke rising from areas in the province's Yarmouk Valley, a route used by Syrian refugees fleeing the fighting to Jordan. The official insisted on anonymity, citing army regulations.

On Saturday, rebels seized several army checkpoints, clearing a 25-kilometer (15-mile) stretch along the Syrian-Jordanian border.

Israel's military said Sunday its soldiers were on routine patrol in the Golan Heights when they were fired upon and responded. It did not say what weaponry was used or specify if those firing from Syria were rebels or government forces.

For the last week, Syrian rebels have been capturing territory at the foot of the Golan Heights, a strategic plateau that Israel captured from Syria in the 1967 Mideast war and later annexed.

The Syrian Observatory also reported clashes in two districts in the Syrian capital, including near the Damascus international airport. It said the army, backed by warplanes, struck at rebel targets in the northern city of Hama.

The U.N. says more than 70,000 people have been killed since the crisis began in March, 2011.

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Halaby reported from Amman, Jordan.