Two senior commanders with the American-backed Free Syrian Army were kidnapped and executed by al Qaeda fighters, highlighting the increasingly violent schism between the country's secular opposition and the Islamic radical groups fighting alongside them.

The two FSA commanders, Mohammad al-Qadi and Ahmad Jahar, were taken by members of Islamic State of Iraq and al-Sham (ISIS), a teaming of al Qaeda's Iraqi cell and affiliated Syrian militant groups, during a cross-border supply run from Turkey into the northern part of the country.

Their bodies were found outside an ISIS-controlled area near the town of Aziz, 20 miles north of the opposition stronghold of Aleppo, according to local news reports.

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The FSA is the largest and most organized of the secular rebel factions battling to topple longtime leader President Bashar Assad. The group's leader, Gen. Salim Idris, also heads the Syrian Military Council (SMC).

The Obama administration has recognized the SMC's political wing, the Syrian National Council, as the legitimate successor to the Assad regime.

Council members are expected to take part in scheduled peace talks with representatives from the Assad government in Geneva in January, led by U.N.-Arab League envoy Lakhdar Brahimi.

The deaths also come weeks after FSA and ISIS commanders reportedly agreed to a cease-fire, in order to focus their attacks on Iranian forces moving into the country.

"ISIS and FSA infighting ceased mostly after a meeting of commanders ... led all parties to decide to focus their efforts" against forces loyal to Assad and his Iranian backers, Syrian Support Group spokesman Dan Layman told The Hill in November.

As a result, both groups have launched "joint counteroffensives" against Hezbollah and Iranian military positions in and around Aleppo, according to Layman, whose D.C.-based organization backs the secular wing of the Syrian opposition.

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But the kidnappings and subsequent execution of the FSA commanders have seemingly put an end to that deal, as both sides step up their efforts to position themselves for a post-Assad Syria.

ISIS aspirations in Syria have drawn serious concern from Obama's national security team, as the White House reportedly continues to move weapons and supplies to opposition forces in the country.

The group's resurgence from safe havens in Iraq and into Syria now represents a "transnational threat network" that could possibly reach from the Mideast to the United States, according to the White House.

That said, administration officials are reaching out to other Islamic fundamentalists groups in the region, to help bring about a peaceful end to the Syrian civil war.

The White House is specifically targeting militant groups unaffiliated with the ISIS and its Syrian offshoot, Jabhat al Nusra, which has taken on a significant role within the ranks of the Syrian rebels over the course of the 2.5-year war there.

The outreach aims to “find out whether these people are worthwhile bringing into the diplomatic process,” a senior U.S. official told The Wall Street Journal in December.

Defense Secretary Chuck Hagel Charles (Chuck) Timothy HagelWhile our foes deploy hypersonic weapons, Washington debates about funding Hillicon Valley: Democrats request counterintelligence briefing | New pressure for election funding | Republicans urge retaliation against Chinese hackers National security leaders, advocacy groups urge Congress to send election funds to states MORE also suggested those groups could play a crucial role in bringing the country's civil war to a close.

"If a diplomatic solution is the responsible approach [in Syria], all parties involved are going to have to be represented in some way" in whatever deal is struck, Hagel said last Wednesday.

Any plausible peace plan to end the ongoing carnage in Syria "cannot be achieved by [limiting] ourselves to narrow strips of interest," Hagel told reporters at the Pentagon.