In a statement released on Tuesday, the department said the group had once been part of the Nusra Front, an al Qaeda affiliate also deemed a "terrorist organization" by the United States, but has "since split and now carries out operations independently."
"However, despite the split it is still openly aligned with ANF (Nusra Front)," the State Department said, citing two suicide bombings Jund al-Aqsa launched in March 2015 and a February 2014 village massacre that killed 40 civilians.
Russian warplanes targeted US, British outpost in Syria: report
The official terrorism designation is one way US officials aim to deny sanctioned groups and individuals access to the US financial system.
"Today's action notifies the US public and the international community that Jund al-Aqsa is actively engaged in terrorism," the state department said.
The hardline Jund al-Aqsa has been one of several rebel groups battling Syrian President Bashar al-Assad and his allies in Hama province, a part of Syria where he has tried to consolidate his grip on power against a more than five-year-old insurgency.
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