Nusra Front fighters in Syria
Nusra Front fighters in SyriaReuters

The leaders of the Islamic State (ISIS) and Syria’s Al-Qaeda affiliate, Al-Nusra Front, have agreed on a plan to stop fighting each other and work together against their opponents, The Associated Press (AP) reported on Thursday.

A high-level Syrian opposition official and a rebel commander told the news agency that the agreement was made in a gathering at a farm house in northern Syria last week.

Such an accord could present new difficulties for Washington's strategy against ISIS, according to AP. While warplanes from a U.S.-led coalition strike terrorists from the air, the Obama administration has counted on arming "moderate" rebel factions to push them back on the ground.

Those rebels, already considered relatively weak and disorganized, would face far stronger opposition if the two heavy-hitting militant groups now are working together.

ISIS and Al-Nusra Front have fought each other bitterly for more than a year to dominate the rebellion against Syrian President Bashar Al-Assad.

Powerful jihadist rebel groups in Syria, including the Islamic Front and Al-Nusra Front, became locked in fierce fighting with ISIS last January.

The fighting broke out after allegations that ISIS brutally abused civilians and other opposition fighters battling to topple Assad.

Tensions were further exacerbated when Al-Nusra and other rebel factions accused ISIS of killing Islamist Front commander Abu Khaled al-Suri in a suicide car bomb attack in February.

The latest clashes come even after Al-Qaeda leader Ayman al-Zawahiri ordered the Al-Nusra Front to halt fighting against ISIS.

AP reported late last month on signs that the two groups appear to have curtailed their feud with informal local truces. Their new agreement, according to the sources in rebel groups opposed to both ISIS and Al-Nusra Front, would involve a promise to stop fighting and team up in attacks in some areas of northern Syria.

A U.S. official with access to intelligence about Syria said the American intelligence community has not seen any indications of a shift in the two groups' strategy, but added that he could not rule out tactical deals on the ground.

The official insisted on anonymity because he said he was not authorized to speak publicly about the subject.