BEIRUT — Al-Qaida formally dissociated itself with the extremist Islamic State in Iraq and Syria on Monday, sealing a rift with the most radical group fighting in both countries that has exposed the shrinking influence of the al-Qaida leadership over a new generation of emerging radicals.

The repudiation came after the failure of repeated efforts by al-Qaida leader Ayman al-Zawahiri to heal a dispute between ISIS and another group, Jabhat al-Nusra, that has erupted in fighting in parts of rebel-held northern Syria.

In a statement posted on jihadi forums, al-Qaida’s General Command said that ISIS “is not a branch of the al-Qaida group . . . does not have an organizational relationship with it and is not the group responsible for their actions.”

The rejection leaves Jabhat al-Nusra, which is widely regarded as more moderate than the hardline ISIS, as al-Qaida’s officially anointed affiliate in Syria, where a multitude of armed groups are battling to overthrow Syrian President Bashar al-Assad and also, in some places, one another.

It also means that al-Qaida no longer has a representative in Iraq, where ISIS originated and where it poses a significant challenge to Prime Minister Nouri al-Maliki’s control over the country.

The statement suggested the notorious intractability of ISIS, the most extreme of the Islamist groups fighting in Syria, was to blame for the break. It cited the importance of consultation and teamwork, qualities that ISIS has ignored in its aggressive expansion across northern Syria since it announced its formation in the country last April.

“Clearly Zawahiri believes that ISIS is a liability to the al-Qaida brand,” said Aaron Zelin, who tracks jihadi movements at the Washington Institute of Near East Policy. “They are not playing nice with other groups and they are acting as a sovereign state, aggravating other rebels and hurting the effort against the regime.”

The group’s brutal tactics, including beheadings, floggings and bans on smoking, music and other perceived un-Islamic behaviors, have incurred the wrath of many ordinary Syrians, culminating a month ago in a widespread revolt against ISIS across northern Syria in which the al-Qaida affiliated Jabhat al-Nusra has fought ISIS alongside more moderate rebels.

But ISIS has fought back, and has managed to retain control of the northeastern province of Raqqa and significant portions of rural Aleppo, putting it in control of much of the area’s resources including its oil fields that help it to operate independently of the al-Qaida leadership, said Aymenn al-Tamimi, who monitors jihadi activity in Iraq and Syria with the Middle East Forum.

Although Jabhat al-Nusra is now the sole al-Qaida affiliate in Syria, ISIS is more aggressively pursuing the al-Qaida agenda of establishing an Islamic caliphate, setting up the institutions of state that enable it to administer the areas it controls, he said.