Syrian militias armed by different parts of the U.S. war machine have begun to fight each other on the plains between the besieged city of Aleppo and the Turkish border, highlighting how little control U.S. intelligence officers and military planners have over the groups they have financed and trained in the bitter five-year-old civil war.

The fighting has intensified over the last two months, as CIA-armed units and Pentagon-armed ones have repeatedly shot at each other while maneuvering through contested territory on the northern outskirts of Aleppo, U.S. officials and rebel leaders have confirmed.

In mid-February, a CIA-armed militia called Fursan al Haq, or Knights of Righteousness, was run out of the town of Marea, about 20 miles north of Aleppo, by Pentagon-backed Syrian Democratic Forces moving in from Kurdish-controlled areas to the east.

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“Any faction that attacks us, regardless from where it gets its support, we will fight it,” Maj. Fares Bayoush, a leader of Fursan al Haq, said in an interview.

Rebel fighters described similar clashes in the town of Azaz, a key transit point for fighters and supplies between Aleppo and the Turkish border, and on March 3 in the Aleppo neighborhood of Sheikh Maqsud.

The attacks by one U.S.-backed group against another come amid continued heavy fighting in Syria and illustrate the difficulty facing U.S. efforts to coordinate among dozens of armed groups that are trying to overthrow the government of President Bashar Assad, fight the Islamic State militant group and battle one another all at the same time.

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“It is an enormous challenge,” said Rep. Adam Schiff (D-Burbank), the top Democrat on the House Intelligence Committee, who described the clashes between U.S.-supported groups as “a fairly new phenomenon.”

“It is part of the three-dimensional chess that is the Syrian battlefield,” he said.

The area in northern Syria around Aleppo, the country's second-largest city, features not only a war between the Assad government and its opponents, but also periodic battles against Islamic State militants, who control much of eastern Syria and also some territory to the northwest of the city, and long-standing tensions among the ethnic groups that inhabit the area, Arabs, Kurds and Turkmen.

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