By Mitchell Prothero, McClatchy Foreign Staff

BEIRUT — A rogue al-Qaida offshoot Thursday shocked its rivals in the Syrian rebellion with a surprise offensive in far southeastern Syria, in an attempt to take control of a strategic border crossing with Iraq as well as rebel-held oilfields in the area.

Residents and rebel commanders in the Syrian border city of Abu Kamal said that fighters from the Islamic State of Iraq and Syria began a surprise offensive at dawn Thursday against positions held by units that include al-Qaida’s official representative in Syria, the Nusra Front, and a handful of independent units. ISIS has been in violent conflict with Nusra and other rebel units since January, but according to witnesses Thursday saw some of the heaviest fighting so far.

“They came out of the desert from (the ISIS stronghold of) Raqaa to the north and used the Iraqi border to attack from the south,” said Abu Mohammed al Bukamali, a spokesman for an independent rebel group in the area. “We have beaten back their advance but they are trying to take control of the city for the border crossing and the oil. We are coordinating with Nusra to defend the city and have taken at least 25 martyrs today alone.”

Although the rebel spokesman, as well as activists with links to the Nusra Front, downplayed the success of the advance, the London-based Syrian Observatory for Human Rights, which is sympathetic to the rebels, said that ISIS had made significant gains.

“Since dawn there has been fierce fighting inside Abu Kamal,” the group said in an online statement. “ISIS is advancing and has taken control of several neighborhoods of the town which were previously held by al Nusra Front and Islamist brigades.”

Abu Kamal and the nearby city of Deir Azour, both held by Nusra and its allies, sit directly between ISIS’ critical Syrian stronghold in the rebel-controlled provincial capital of Raqaa and the Iraq province of Anbar, where ISIS is conducting an offensive against the Iraq government for control of the cities of Fallujah and Ramadi. With ISIS and Nusra effectively at war since January, the control of the highway that links Raqaa to Anbar has become a strategic priority.

Despite the ISIS gains, the border crossing remained in the hands of a rebel brigade loyal to the Syrian Military Council, the U.S.-backed rebel coalition, according to witnesses who spoke with wire services.

“We are still controlling the border crossing,” a rebel commander told Agence France-Presse on condition of anonymity.

“ISIS are attacking our positions in Abu Kamal, and the battles since (late Wednesday) are in three different areas about 15 kilometers (10 miles) from the center of the city,” the commander said. “The battles are still continuing.”

ISIS and Nusra had jointly controlled Abu Kamal and Deir Azour prior to January’s brutal infighting after ISIS’ repeated confrontations with fellow rebels finally led the group to renounce all their former comrades, including their one-time commanders in al-Qaida. ISIS was completely driven out of the area in January but wants control of both the crossing and the area’s substantial oil facilities.

“This is the most sustained ISIS assault I’ve seen in the eastern parts of the province,” said Aymeen al Tamimi, who studies Iraq and Syrian jihadist groups for the Middle East Forum. “Without a strong foothold in Deir Azzor’s eastern areas, there’s no link with Anbar province.”

The Syrian regime itself controls only one official border crossing with Iraq — the rest are in the hands of rebels or Kurdish militias that are playing an increasingly autonomous and independent role in the civil war.

Photo Credit: AFP/Abdurashid Abdulle