U.S. Secretary of State John Kerry attends a "Friends of Syria" group meeting hosted by Turkish Foreign Minister Ahmet Davutoglu in Istanbul on April 20. The U.S. plans to double its aid to the Syrian opposition, an official said. (Reuters)

The Obama administration pledged early Sunday to double its nonlethal assistance to the Syrian opposition, to $250 million, after a marathon negotiating session here in which all major international backers pledged to keep their aid to the rebels out of extremist hands.

The 11 countries that are the main opposition supporters, including Britain, France and major Middle Eastern powers, in addition to the United States, agreed in a joint statement to “channel all military assistance exclusively through the SMC,” the Supreme Military Council that is the military equivalent of the U.S.-backed political opposition group.

The statement, Kerry said, brings “everybody on the same page with respect to how assistance will be provided.”

The agreement, if it holds, would mark a significant effort to stem the escalating fighting power and influence of al-Qaeda-linked groups, including Jabhat al-Nusra, that have joined the two-year effort to oust Syrian President Bashar al-Assad.

Assad’s political opponents have welcomed them to the fight, arguing that getting rid of Assad and stopping the carnage that has left more than 70,000 dead and led nearly 5 million to flee their homes was more immediately important than ensuring that Islamic extremists were barred from participating.

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The dispute over which rebel factions to aid, what kind of assistance to provide and what to demand in return was matched by divisions within the political opposition itself, where different factions — supported by different foreign governments — were vying for power and control over Syria’s future.

In a separate statement, the political Syrian Opposition Coalition acknowledged “that there are radical/extremist elements in Syria which follow an agenda of their own. We firmly reject and condemn all forms of terrorism and any extremist ideology.”

The meeting, designed to forge greater unity within the coalition, had become an extended argument Saturday night about what one official called the “competing agendas” among the supporters. Some countries, including Qatar and Turkey, favor ongoing dominance of Syria’s Muslim Brotherhood within the coalition, and have been less concerned than others that military assistance is flowing to Islamist extremists such as the Jabhat al-Nusra group, according to officials who spoke on condition of anonymity about the sensitive talks.

Among the many subdivisions is that, while Saudi Arabia has been at odds with Qatar over the latter’s approval of the Muslim brotherhood, the Saudis have been less strident than many in the West over aid to the extremists. The Saudis and Qatar are the two countries known to be supplying lethal weaponry to the opposition.

Western governments, and some in the Middle East, who have been leery of aiding the coalition until moderates are more clearly in a dominant position, have argued that the donor group needs to spend more energy pushing opposition political and military leaders to unify their message and activities, including the provision of services to beleaguered populations in rebel-controlled areas, and outreach to minority groups who continue to side with Syrian President Bashar al-Assad.

One Western official said that Kerry, who is leading the U.S. delegation here, was outspoken in pushing the coalition to make strong and verifiable commitments to pluralism and minority rights.

Coalition President Moaz al-Khabit, accompanied by rebel military leader Gen. Salem Isriss, initially presented a list of demands, including establishment of a no-fly zone to protect civilians from air and missile attacks, a U.N. Security Council resolution that would condemn Assad’s air attacks and possible use of chemical weapons, and providing more military and civilian aid.

While the United States and others said they were willing to spend more, they have made it clear that they will not intervene militarily, either on the ground or in the air. U.N. resolutions have been repeatedly blocked by Russia.

Both the supporters and the opposition worked into the night, hours past their original deadline, to produce a communique that officials said unified both the moderate opposition and its international backers.

British Foreign Secretary William Hague said the opposition had pledged their commitment to pluralism and protection of democracy in “the clearest language they have ever used,” and that more “important announcements” would be coming from international backers.

The United States has given food and medical assistance to the opposition military. Kerry did not specify what would be authorized under the new assistance, but administration officials have said that President Obama has approved items such as body armor and night-vision goggles.

The United States has refused to supply weapons requested by the opposition, saying that it is assisting those who are sending lethal equipment in vetting the recipients.

The State Department has been the most ardent proponent within the administration of increasing support to the rebel military, and a consensus by the gathering here may help convince a skeptical White House to go even further.

While in Istanbul, Kerry will also meet with Turkish Foreign Minister Ahmet Davutoglu. In addition to Syria, administration officials’ discussions will focus on Kerry’s efforts to forge a rapprochement between Turkey and Israel after a breakdown in relations following Israel’s fatal 2010 attack on a Turkish aid flotilla en route to the Gaza Strip.

A senior State Department official traveling with Kerry said that the two sides would meet next week for discussions on Israeli compensation to victims of the attack. “Hopefully, that would be followed by both countries sending ambassadors to the other capital,” the official said, speaking on the condition of anonymity about the still-tenuous agreements.

Late Sunday, Kerry will travel to Brussels for meetings with NATO foreign ministers. Those discussions are expected to focus on Afghanistan as well as Syria. Kerry will also meet there with his counterpart from Russia, which has continued to back Assad in Syria and to block U.N. actions there.