Bhutto died during surgery from injuries sustained in a suicide bomb attack, which occurred when shots were fired by an assassin, who then blew himself up as Bhutto left an election rally in Rawalpindi, near Pakistan's capital city of Islamabad.
Initial reports said around 20 people were killed and at least 15 injured in the blast.
"She was a loyal supporter of America and pledged to defeat mujahideen and for that she was eliminated," Pakistani television quoted notorious al-Qaeda field commander in Afghanistan Mustafa Abu al-Yazid as saying in a satellite phone call.
Al-Yazid, an Egyptian national, also known as Sheikh Saeed, is an original member of al-Qaeda's Shura leadership council and has been a trusted adviser to Osama bin Laden for more than a decade.
No official comment on the report has so far followed.
Bhutto, 54, who had twice been the country's prime minister, was about to run in parliamentary elections January 8 as leader of the Pakistan People's Party (PPP).
She returned to Pakistan in October after more than eight years of self-imposed exile. Her arrival was overshadowed by a terrorist act which took the lives of 140 people and injured 500 as hundreds of thousands of people lined the streets to greet her homecoming.
She had criticized the government of incumbent Pakistani President Pervez Musharraf for failing to take strong action against al-Qaeda and vowed to bring stability to the country amid the recent surge of Islamist violence.
Bhutto was pronounced dead late on Thursday in a hospital in Rawalpindi and her body arrived in her family village in Sindh province for burial on Friday, Reuters reported.
Meanwhile, members of the U.N. Security Council have voted unanimously to condemn the assassination of the Pakistani opposition leader and urged all nations to help bring those responsible for "this reprehensible act" to justice amid fears that the terrorist act may destabilize the situation in the country.
Russian President Vladimir Putin said on Thursday he hoped those responsible for the assassination of Pakistan's opposition leader Benazir Bhutto will be found and punished.
"Forces of terrorism are challenging not only Pakistan, but the international community. We hope that the organizers of this crime will be found and appropriately punished," Putin said in a statement addressed to the Pakistan leader, Pervez Musharraf.