(CNN) Recent intelligence indicates that al Qaeda leader Ayman al-Zawahiri has a "heart complaint," according to a senior official involved in international counterterrorism efforts.

The official said the information suggests Zawahiri had a potentially serious condition but cautioned it was difficult to ascertain the severity of his health problems and what effect they might have on his longevity.

The possibility that Zawahiri, who just turned 68, is seriously ill increases uncertainty over al Qaeda's long-term leadership succession plans. Osama bin Laden's son Hamza bin Laden had been widely tipped to one day take over command of al Qaeda but a US official told CNN on Wednesday that the US now believes that he is dead

Zawahiri's health was also referenced in a UN monitoring report that was circulated this summer to the Security Council. Citing "member state information," the report noted Zawahiri was "reported to be in poor health." It stated that although "Al-Qaida remains resilient" the "health and longevity of its leader ... and how the succession will work are in doubt."

Zawahiri has continued to frequently appear in al Qaeda videos and was last heard from last month. There were no obvious signs that he has a serious health condition in these videos.

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