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(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.

The US still sees al Qaeda as a major threat and a US intelligence assessment on the terror group released in January stated that senior leaders are “strengthening the network’s global command structure” as part of its effort to inspire and encourage attacks against the West.

The group’s “command and control is agile, active and able to respond,” one US official who tracks the group told CNN earlier this year.

The first US official said the US had a role in Hamza bin Laden’s death but did not provide details. The official added that the US government had recently received evidence that it believes corroborates his death.

The New York Times reported that Hamza bin Laden had been killed in an operation within the last two years and at some point before the US offered a $1 million reward in February for information about his whereabouts. The last time he was heard from was on March 31, 2018, when al Qaeda released a tape of him discussing American-Saudi relations.

According to the senior official involved in international counterterrorism efforts, there is intelligence that Hamza bin Laden was most recently based in the Afghanistan-Pakistan border region and met with Zawahiri there during the last two years. The official cautioned that the intelligence picture on bin Laden’s movements was fragmentary and imprecise.

The official said that Hamza bin Laden, with his reputation for eloquence in jihadi circles, had in recent years built up credibility as a spokesman for al Qaeda and unlike Zawahiri had notably held back from criticizing ISIS to avoid burning bridges. But while he may have been trying to position himself to eventually fill his father’s shoes as the head of al Qaeda, the official did not believe that he was seen by the al Qaeda leadership as a candidate to immediately take over if Zawahiri died or were incapacitated.

Counterterrorism experts believe Egyptian jihadi veterans Abu Muhammad al-Masri and Saif al-Adel are likely next in line to take over from Zawahiri. Both are wanted in connection with al Qaeda’s bombings of two US embassies in east Africa in 1998. Al-Masri (who the US has designated as a wanted terrorist under the name Abdullah Ahmed Abdullah) is Hamza bin Laden’s father-in-law.