Free Syrian Army fighters are seen together in Wadi Al-Dayf in the southern Idlib countryside July 16, 2014. Reuters/Khalil Ashawi

Moderate and Islamist rebel groups in a Damascus suburb in Syria signed a “non-aggression” agreement with the Islamic State (ISIS), a Syrian monitoring group said. Some of the brigades are part of the U.S.-backed Syrian Revolutionary Front but have now agreed to a truce in Hajar al-Assad with ISIS in order to take down Syrian President Bashar Assad, according to the Agence-France Press. The shifting alliances highlight the challenge faced by the Obama administration in its fight against ISIS in Syria: Even moderates supported by the U.S. can't be relied on to help battle the terrorist group.

"The two parties will respect a truce until a final solution is found and they promise not to attack each other because they consider the principal enemy to be the Nussayri regime," the U.K-based Syrian Observatory for Human Rights reported. (Nussayri is the pejorative term used for Assad’s Alawite regime.)

The non-aggression pact came days after President Barack Obama announced his strategy to attack the militant group formerly known as the Islamic State of Iraq and Syria “wherever they are.” The president said the U.S. would lead an international coalition in an air campaign against Islamic State strongholds in Syria as well as asking Congress for additional funding to train and arm moderate rebels on the ground.

"We must strengthen the opposition as the best counterweight to extremists like ISIL, while pursuing the political solution necessary to solve Syria's crisis once and for all," Obama said on Wednesday.

The Observatory did not immediately confirm which group had signed the agreement, but many reports said it was a group that belonged to Western-backed Supreme Military Council (SMC) of the Free Syrian Army -- in other words, the moderate rebels.

The moderate rebels are a group of brigades that fall under Syrian National Coalition, of which the Free Syrian Army is a member. The group's main goal is to take down the Assad regime. In some regions of Syria, ISIS has impeded the rebels’ ability to fight against regime forces, which would make ISIS their enemy as well. The Assad regime is a far more important enemy than ISIS or al Qaeda, the FSA reportedly said in a statement released on YouTube.

“The priority is the regime,” Ziad Obeid, the moderate rebel commander in Aleppo, told the New York Times Thursday over Skype. “But it is ISIS that is preventing any progress on the ground, so we have to get rid of it, too.”

Alliances have changed drastically over the course of Syria’s more than three year long civil war. At the beginning, the lines between rebels and regime forces were fairly clear, and ISIS was squarely on the rebel side. Later, moderate groups began clashing with extremist rebel groups, and when al Qaeda’s affiliate in Syria, Jabhat al-Nusra, disowned ISIS, the militant group was left with very few allies.

When ISIS began making significant gains in the country and eventually expanded into Iraq, regime forces looked the other way, if only because the two now shared an enemy: the moderates. Yet once ISIS declared itself a Caliphate under the name Islamic State, many of those anti-Assad moderates began to pledge allegiance to them or other extremist groups.

Earlier this week three leaders of the al Qaeda linked Ahrar al-Sham Brigade were killed in an attack. On Thursday the group named Abu Jaber it new leader. Jaber had previously fought along two rebel groups, Al Fajr Islamic Movement and the Musab Bin Umayr Battalion, which used to be part of the FSA and later defected to the Islamic Front, an umbrella group of extremist brigades not associated with the moderate FSA, according to the Long War Journal.

The Dawood Brigade in Al-Bab Syria is another example of the quickly shifting allegiances. It used to be part of the Free Syrian Army. But it recently defected to ISIS and reportedly gave a hostage, U.S. journalist James Foley, to the militants as a token of its loyalty.