The BBC reported officials said Rabbani had been meeting with two members of the Taliban when the explosion occurred, but it remained unclear whether they were involved in the attack.
Ministry of Interior sources told The Daily Telegraph two suicide bombers carried out the attack at Rabbani's home in the upscale Wazir Akbar Khan neighborhood near the U.S. Embassy.
Government sources said they believed two "fighters from an unknown group" were behind the attack but no one had claimed responsibility, the Telegraph said.
Rabbani, Afghanistan's president from 1992 until his ouster by the Taliban in 1996, was chairman of the Afghan High Peace Council, which leads Afghan efforts to negotiate with the Taliban, since last year.
The Telegraph said four other members of the High Peace Council were also killed and a senior adviser to President Hamid Karzai, Mohammed Masoon Stanekzai, was seriously injured.
Michael Semple, a Taliban expert and former deputy EU special representative to Afghanistan, described the attack as "one of the biggest blows the peace process in Afghanistan has faced," the Telegraph said.
The attack, Semple said, "raises the question of whether there is the space for Afghans to talk and whether they can avoid a civil war, but it is also important to understand who did this because it is clearly people who feel threatened by a peace process."
Semple, who knew Rabbani, said Pakistani militants in Waziristan may be responsible for the attack.
Rabbani, an ethnic Tajik, had been a mujahedin leader praised for his pivotal role in ending the Soviet occupation of Afghanistan in the 1980s and 1990s.
After being ousted as president, he served as the nominal head of the Northern Alliance, mostly minority Tajiks and Uzbeks, who took power after the Taliban's fall.