(CNN) -- Al Qaeda says its No. 3 man and commander of its operations in Afghanistan has died, according to a group that monitors Islamist websites.
Al Qaeda announced the death of Mustafa Abu Yazid in a message posted on such websites Monday, the Maryland-based SITE Intelligence Group said. While the message didn't detail the circumstances of Yazid's death, it did say his wife, three of his daughters, a granddaughter and others were killed, according to SITE.
Yazid, an Egyptian national, had a "more expansive portfolio" than previous No. 3 officials in the terror group, according to a U.S. official who asked that he not be named since he is not authorized to speak on the record.
As al Qaeda's CEO, he was important to al Qaeda's finances and has been recently involved in the facilitation of terror plots, the U.S. official said. He was a founding member of the group and served on its leadership council, the official said.
"His leaving the scene is nothing short of significant," the official said.
His death is a near-term blow to al Qaeda as he was the chief conduit to leaders Osama bin Laden and Ayman al-Zawahiri, the official said.
Because of its degraded capabilities, al Qaeda has found itself in the position of needing to integrate its efforts with other extremist groups such as the Pakistani Taliban and the Afghan Taliban, the U.S. official said. Yazid helped forge relationships with those other extremist groups in Pakistan, Afghanistan and elsewhere, the official said.
Yazid is believed to have been a co-founder of the terrorist group, according to SITE.
A U.S. official speaking on condition of anonymity said the government has "strong reason to believe" that Yazid was killed recently in Pakistan's tribal areas.
"In terms of counterterrorism, this would be a big victory," the official said, noting that Yazid was al Qaeda's prime conduit to leader Osama bin Laden and his deputy, Ayman al-Zawahiri.
Yazid served prison time with al-Zawahiri for their involvement in the 1981 assassination of Egyptian President Anwar Sadat, the U.S. official said.
U.S. intelligence officials said Yazid would brief would-be al Qaeda operatives who came from the United States for attacks abroad.
In the past, Yazid has made numerous statements on jihadist websites and in video releases from al Qaeda's media arm, al Sahab.
Most recently, he announced that al Qaeda took responsibility for a December 30 attack on a base in eastern Afghanistan that killed seven CIA officers and consultants and a Jordanian army captain. He said the attack avenged the death of Baitullah Mehsud, leader of the Taliban in Pakistan who was killed in a missile strike in August, and al Qaeda operatives Saleh al-Somali and Abdullah al-Libi.
IntelCenter, a think tank that tracks terrorist groups, said Yazid's death was one of the most significant blows against al Qaeda in recent years.
But IntelCenter added that al Qaeda knows losses are inevitable and plans for them.
"While the loss of al-Yazid will have an impact, the group will likely maintain its operational tempo in terms of attacks and other activities," IntelCenter said in a statement.
Yazid rose to the No. 3 position when Abu Laith al-Libbi was killed in 2008, the U.S. official said.
Previous al Qaeda No. 3 officials to die on the job in the last decade include: Mohamed Atef (killed in 2001), Khalid Sheikh Mohammed (captured in 2003), Abu Farraj al-Libbi (captured in 2005) and Hamza Rabia (killed in 2005), the U.S. official said.
CNN's Nic Robertson in Kabul, Afghanistan, and Pam Benson contributed to this report.