DAMASCUS — Syria’s al-Qaeda wing has pledged loyalty to the rival Islamic State in Iraq and al-Sham (ISIS) group in a Syrian border town, a monitoring group said, boosting ISIS’s control on both sides of the frontier.

Fighters from Nusra Front, the Syrian wing of al-Qaeda, took an oath of allegiance to ISIS in the town of Albu Kamal, close to the Iraqi border, the British-based Syrian Observatory for Human Rights and an Islamist website said on Wednesday, June 25.

The central leadership of al-Qaeda has disowned ISIS and proclaimed the Nusra Front as its official Syrian affiliate.

Controlling parts of Syria’s Deir al-Zor province, where Albu Kamal is located, could help ISIS link up its territorial gains across Syria and Iraq, where it overran the main northern city of Mosul on June 10.

Twitter users posted a photo they said showed the Nusra Front leader of Albu Kamal, Abu Yusuf al-Masri, swearing loyalty to one of ISIS’s prominent fighters.

“It is very important because Nusra is strong in Albu Kamal,” the Observatory’s Rami Abdurrahman said. “We cannot say (ISIS) controls Albu Kamal but we can say they are now in Albu Kamal.”

While ISIS and Nusra Front have linked up in Albu Kamal, further north there were violent clashes between the two groups, underlining how the conflict shifts from town to town in Deir al-Zor, Syria’s main oil-producing region.

The groups fought each other in Shahil, a Nusra Front stronghold located north of Albu Kamal on the Euphrates River, the Observatory said, adding that many Nusra leaders originated from the town.

Last week, ISIS captured three key towns in the area, which runs along the Euphrates River linking Syria and Iraq. The towns are close to Deir al-Zor’s military airport, which is still under the control of the Syrian army.

While some towns in Deir al-Zor have been seized after deadly battles with rival groups, others have been won over by ISIS without a fight through a mixture of coercion and alliances.