Masood (centre) is a veteran of the Afghan conflict
Mr Masood was injured in a bomb explosion near his home in the north of the country.
"We believe he's dead," the official said, without detailing what he based the assertion on.
Conflicting reports
But US State Department spokesman Philip Reeker said he could not confirm the news of Mr Massoud's death.
"We are not able to confirm any of [the reports] at this point, but I am able to say that we are sorry to see this attempt on the life of a key factional leader in Afghanistan," Mr Reeker said.
"This could set back the search for a peaceful settlement of the decades-old war."
A report in the Russian news agency, Tass, quoted unidentified sources in the Tajikistan capital Dushanbe as saying that Mr Masood had died of his wounds.
But other reports quote Afghan opposition sources as saying that he was recovering in hospital.
"Thank God he is in good health - I talked with him through a wireless this morning and he was OK," the French news agency, AFP, quoted an opposition spokesman, Waisuddin Salik, as saying.
Attack
Mr Masood was giving an interview to two Arab journalists in Afghanistan's northern Takhar province when a bomb went off. It had been concealed in a video camera.
A spokesman for Mr Masood, quoted by AFP, said the blast "killed the two Arabs and slightly injured the commander".
He said Mr Masood sustained "minor" injuries and his health was "not giving cause for concern".
A close aide was also reported killed in the blast.
According to earlier Russian news agency reports, Mr Masood was injured when his vehicle was hit by an explosion near the frontline.
'Lion of Panjshir'
Mr Masood, known as the "Lion of the Panjshir" is a veteran commander of the opposition Northern Alliance. His forces remain loyal to the ousted government of Burhanuddin Rabbani.
The 49-year-old ethnic Tajik commander is widely regarded as the last bulwark against the ruling Taleban.
In the past year, Taleban forces have pushed north and now control some 90% of Afghanistan. Both sides are trying to gain and secure ground before the winter sets in.
Heavy fighting has taken place recently in Takhar and in areas close to the Tajik border.
Mr Masood played a major role in the Afghan resistance to Soviet occupation in 1979-1989.