Kabul Police Chief Ayub Salangi told the BBC during the weekend that Maulawi Arsala Rahmani was assassinated while on his way to his Senate office.
Rahmani was a member of the Afghan senate representing the eastern province of Paktika. He was a member of President Hamid Karzai's peace council, founded to open dialogue with the Taliban.
The U.N. Assistance Mission in Afghanistan, in a statement, said the assassination would do little to undermine a national reconciliation effort under way in the country.
"UNAMA remains committed to supporting the Afghan-led peace and reconciliation process and providing requested assistance to the High Peace Council," the mission said in a statement.
Rahmani was a former senior figure in the Taliban who had since reconciled with the Afghan government.
His assassination follows the early May launch of a spring offensive by the Taliban, which ruled Afghanistan prior to the U.S.-led invasion in 2001.
NATO representatives in Chicago this week are expected to outline their future role in Afghanistan. National forces are slowly taking the lead in security operations in the country as international forces wind down their mission.