Jihadist fighters of Al-Nusra Front have seized control of a checkpoint on northeast Syria's border with Iraq, the Syrian Observatory for Human Rights said on Friday.
The rebels overran the post at Yaarubiyeh in oil-rich Hassakeh province on Thursday after fierce clashes with government forces, the Britain-based Observatory said in a statement.
The border crossing had been controlled by rebel forces last year before the army recaptured it.
Al-Nusra Front, an increasingly influential factor in Syria's conflict, also took control of the nearby town of Shaddadeh and surrounding villages in mid-February.
It first gained notoriety for its suicide bombings in Syria's main cities and has evolved into a strong fighting force leading attacks on battlefronts throughout the country.
The Front's tactics and suspected affiliation to Al-Qaeda's offshoot in Iraq have landed it on the US list of terrorist organisations.
Rebels of the mainstream Free Syrian Army have told AFP that Al-Nusra fighters, despite their small numbers, have greater economic and logistical support than other insurgents and funding from abroad.
The jihadist group targets strategic points mainly in eastern Syria, including oil and gas facilities, and draws recruits from the local population by paying them a salary.
It aspires to create an Islamic state in Syria, whose regime accuses Saudi Arabia and Qatar of financing Islamist fighters in the two-year-old conflict that the UN says has killed at least 70,000 people.