A senior figure in an al Qaeda-linked group in Syria has been killed in a bloody dispute with rival rebel factions that has raged for more than three weeks across opposition-held parts of the country, activists and an Iraqi intelligence official said Monday.

Heavy fighting between the Islamic State of Iraq and the Levant and rival rebels has left more than 1,400 people dead across northern Syria since Jan. 3. The clashes, which erupted after months of growing resentment against the Islamic State's heavy-handed tactics in the territory under its control, are the most serious among the opponents of Syrian President Bashar Assad since the country's civil war began.

The senior militant, an Iraqi who went by the nom de guerre of Haji Bakr, was killed earlier this month in the town of Tal Rifaat in the northern province of Aleppo, according to the Britain-based Syrian Observatory for Human Rights monitoring group. That corresponded to reports provided by two activists based in the city of Aleppo, Hassan Kattan and another who spoke on condition of anonymity for security reasons.

A senior Iraqi intelligence official who specializes in al Qaeda said Haji Bakr was assassinated in a killing tied to the dispute between the Islamic State and other armed opposition groups.

The official, who spoke on condition of anonymity because he was not authorized to brief the media, said Haji Bakr's real name was Sameer Abid Mohammed al-Halefawi, and that he served as an air-defense officer in Saddam Hussein's army before joining al Qaeda in Iraq after the 2003 U.S.-led invasion.

Haji Bakr supervised a series of attacks against hotels and embassies in Iraq in 2010, and headed the military council of al Qaeda in Iraq after the group's then-leader, Abu Omar al-Baghdadi, was killed in 2010, the official said.

As Syria's uprising, which began in March 2011, slowly evolved into an insurgency, al Qaeda's Iraqi branch established an affiliate in Syria, the Nusra Front, providing it with weapons, money and manpower. Over time, the Nusra Front rose to become one of the most powerful groups in Syria's armed opposition.

But the groups had a falling out in April when the head of al Qaeda in Iraq announced his group's merger with the Nusra Front to form the so-called Islamic State of Iraq and the Levant. The leader of the Nusra Front rejected the merger, and has continued to pursue its own independent operations. The Islamic State, meanwhile, has largely eclipsed the Nusra Front in many parts of northern Syria.

Haji Bakr had almost no public profile until a few weeks ago as rumors spread that he had been killed.

Aymenn al-Tamimi, an expert on militant groups at the Middle East Forum, said it's not surprising that Haji Bakr was little known, saying the Islamic State "is very low profile about who's at the very top of their leadership."

"You could ask people within ISIS and they couldn't tell you," he said, using an acronym for the Islamic State.

Also Monday, the international mission ridding Syria of its chemical weapons said a second batch of toxic chemicals has been removed from the country.

The United Nations and the Organization for the Prohibition of Chemical Weapons said in a joint statement that the materials were verified before being loaded onto Danish and Norwegian ships and taken out of the country. The vessels were accompanied by a naval escort provided by China, Denmark, Norway and Russia, the mission said.

U.N. spokesman Khaled al-Masri in Damascus said it was the second shipment of chemicals to leave Syria. The first batch of poison gas precursor agents was removed nearly three weeks ago.

The most dangerous chemicals in Syria's stockpile were supposed to have been removed by Dec. 31, but poor security, bad weather and other factors meant the mission missed that deadline.

Meanwhile, a U.S. cargo ship loaded with sophisticated equipment is setting sail Monday for the Mediterranean Sea where it will be used to destroy Syria's chemical weapons. The MV Cape Ray was slated to leave in the afternoon for what is expected to be a roughly two-week trip to the Italian port of Gioia Tauro, where chemicals will be transferred to the ship to be destroyed on board at sea.

The U.S. and Russia brokered an agreement to rid Syria of its chemical weapons following a deadly chemical attack on rebel-held suburbs of Damascus in August.