FILE - In this file photo taken Monday, June 23, 2014, fighters from the Islamic State group parade in a commandeered Iraqi security forces armored vehicle down a main road at the northern city of Mosul, Iraq. The Islamic State group is often described as the most fearsome jihadi outfit of all: a global menace outweighing al-Qaida, with armies trembling before its advance. But while the group has been successful at seizing parts of Iraq and Syria, it is no unstoppable juggernaut. (AP Photo, File)

As the United States begins to deepen ties with moderate Syrian rebels to combat the extremist group ISIS, also known as the Islamic State, a key component of its coalition appears to have struck a non-aggression pact with the group.

According to Agence France-Presse, ISIS and a number of moderate and hard-line rebel groups have agreed not to fight each other so that they can focus on taking down the regime of Syrian President Bashar Assad. Other sources say the signatories include a major U.S. ally linked to the Free Syrian Army. Moreover, the leader of the Free Syrian Army said Saturday that the group would not take part in U.S. plans for destroying the Islamic State until it got assurances on toppling Assad.

The deal between ISIS and the moderate Syrian groups casts doubt over President Barack Obama's freshly announced strategy to arm and train the groups against ISIS.

The AFP report cited information from the Syrian Observatory for Human Rights, a U.K.-based group monitoring the Syrian civil war, which said parties to the agreement "promise not to attack each other because they consider the principal enemy to be the Nussayri regime." The term Nussayri refers to the Alawite ethnic group that Assad and many of his supporters belong to. AFP said the agreement was signed in a suburb of the Syrian capital, where ISIS has a strong presence.

Charles Lister, a fellow at the Brookings Institution's Doha Center, cited a report from the anti-regime Orient Net website to suggest on Twitter that the signatories of the ceasefire include a U.S.-backed coalition called the Syrian Revolutionary Front. According to the U.K.-based outlet Middle East Eye, that same Orient Net report says the ceasefire between groups described in the U.S. as "moderate rebels" and the Islamic State was mediated by the al-Nusra Front, al Qaeda's affiliate in Syria.

As recently as March, the Syrian Revolutionary Front and its leader were described in Foreign Policy as "the West's best fighting chance against Syria's Islamist armies." As of that report, the group controlled 25,000 fighters and its leader had close ties with the Western-friendly Syrian National Coalition.

Its leader initially won Western favor by successfully fighting ISIS in northern Syria.

"He proved his mettle in a sense and that's what endeared him to the Americans," said Joshua Landis, a prominent Syria expert at the University of Oklahoma. "The Americans are looking for people who can actually fight. That's been their problem: they've gone with people who are moderate but they don't know to fight. This guy appears to be both moderate and he knows how to fight."

The Orient Net report on the ceasefire identified the Syrian Revolutionary Front as part of the Free Syrian Army, the loose array of non-jihadist rebel brigades that the U.S. has directly supported since last year. Obama asked Congress to approve $500 million to train and equip "vetted" Syrian rebels this summer. He repeated his request in his address Wednesday about ISIS.

Despite its reputation as a palatable ally, the U.S.-backed Syrian Revolutionary Front has previously said that its chief goal is not to stop the rise of extremists, but to topple Assad. In April, its leader told The Independent, "It’s clear that I’m not fighting against al-Qa’ida. This is a problem outside of Syria’s border, so it’s not our problem. I don’t have a problem with anyone who fights against the regime inside Syria."

The prospect of a group once supported by the U.S. now sitting down with ISIS raises fundamental questions about U.S. strategy in Syria. Why support Syrians who have a very different, clearly stated goal and who will act as they see fit to achieve it? What assurance does the administration have that fighters it trains and arms in Syria won't ally with ISIS if it seems like the most effective anti-Assad force?

The White House argues that its ability to spotlight and support reliable rebel groups has been heightened by improved and expanded intelligence. In an interview with The Huffington Post before news of the pact broke, deputy national security adviser Ben Rhodes expressed confidence in U.S. allies in the region.

"We have been working with the Syrian opposition now for a couple of years, providing them assistance, non-lethal at first but then we [now] provide them with some military assistance, so we know them better today than we did a year, two years ago," Rhodes said. "There are people who have been vetted who we have relationships with, who we deliver assistance to, so we're not starting from scratch."

Many of those groups, the administration acknowledges, have not passed a vetting process, which explains the delay in expanding assistance. But the news that the Syrian Revolutionary Front, a major player in the moderate coalition, has now chosen to stop fighting ISIS may inspire other groups, either already vetted or still waiting for aid, to determine that a deal with the extremist group is worthwhile. Given reports that Assad avoided fighting ISIS in order to crush the moderate rebels -- his calculus being that the West would eventually combat the extremists, as it is now doing -- potential U.S. partners may decide that instead of being prey to both extremists and the government, they should settle one battle.

"These guys are all starved for arms," Landis said. "They don’t want to go get themselves killed by fighting ISIS until they figure out where Obama is."

That turns a conflict that the White House hopes is three-sided -- with radical Sunnis, moderate Sunnis and Assad all battling each other -- into a sectarian, two-sided war of Sunnis against Assad. Reports already suggest that Syrians who entered the civil war opposing Assad are now turning to ISIS as their best bet for a different kind of government.

Rhodes warned that a wrong move by the U.S. may lead to that precise perception and reality.

"If we were to try to run a play with Assad, we would ensure that they" -- all Sunni rebel groups -- "were turned against us, and in fact we would be taking sides in a sectarian war against one side. We need a Sunni partner in these countries," he said. "That's why we need this inclusive government [in Iraq] and that's why we need a Sunni opposition partner in Syria."

This news suggests that partners will be hard to find. Lister said the pact is a product of failed U.S.-led Western policy in Syria.

"This underlines serious frustration w. lack of US-backing to [Free Syrian Army] opposition in fight vs Assad," he tweeted.

If true, Landis said, the news of a ceasefire proves Washington does not know who it can support or trust within the fractured country.

"We don't know who the moderates are," Landis said. Describing a recent interview in which a Free Syrian Army commander told an Arab outlet that the U.S. wanted to make Syrian rebels "slaves," he added, "These guys are supposed to be our buddies?"

UPDATE: September 14 --The Hill reported Sunday that, according to a Syrian National Coalition official, no U.S.-vetted Syrian opposition groups have entered a ceasefire agreement with the Islamic State. However, the official said he could not speak on behalf opposition groups that have not been vetted by U.S. officials.