BEIRUT: Al-Qaeda's Syrian offshoot Wednesday made an oath of loyalty to the Islamic State of Iraq and Greater Syria (ISIS) at a key town on the Iraqi border, a monitor said.

The Syrian Observatory for Human Rights said that the merger is significant because it opens the way for ISIS to take control of both sides of the border at Albu Kamal in Syria and Al-Qaim in Iraq, where the jihadist group has led a major offensive this month.

After months of clashes between the two sides, Al-Qaeda's official Syrian arm the Nusra Front "pledged loyalty to ISIS" in Albu Kamal, said Observatory director Rami Abdel Rahman.

"The pledge comes amid advances by ISIS in Deir al-Zor province" in eastern Syria on the Iraqi border, Abdel Rahman told AFP.

An ISIS jihadist confirmed the reports on Twitter, and posted a photograph showing an Egyptian Nusra Front commander shaking hands with an ISIS leader of Chechen origin.

Although both ISIS and the Nusra Front are rooted in Al-Qaeda, the two have been rivals for much of the time that ISIS has been involved in Syria's civil war since spring last year.

"They are rivals, but both groups are jihadist and extremists. This move will create tension now with other rebel groups, including Islamists, in the area," said Abdel Rahman.

An activist in Albu Kamal told AFP via the Internet that "there is a lot of tension, and the situation is only going to get worse."

Using a pseudonym for security reasons, Hadi Salameh also said that the merger "will cause a big problem with the local tribes, who will not welcome this change."

Another activist said that the merger comes days after local rebel brigades who had been working with Nusra Front signed a declaration excluding the official Al-Qaeda branch from the Islamic court, which acts as the de facto power in many rebel areas of Syria.

"The loyalty oath (to ISIS) comes after tension between Nusra and the local rebels," said the activist, Abdel Salam al-Hussein.

Meanwhile, the Syrian air force carried out air raids targeting ISIS-controlled Raqa in the north of the country and Muhassen in the east.

President Bashar Assad's regime has rarely targeted ISIS bastions, except in recent days after the group and other Sunni militants launched an offensive in Iraq, wresting control of Mosul and other parts of Iraq.

ISIS aspires to create an Islamic state that straddles Iraq and Syria.