KABUL, Afghanistan â€"  A gunman shot dead a top member of the Afghan peace council Sunday in Kabul, police said. The assassination strikes another blow to efforts to negotiate a political resolution to the decade-long war.

Arsala Rahmani, a former Taliban official turned Afghan peace negotiator, was in his vehicle when he was killed by an unknown attacker in another vehicle at an intersection in the west part of the city, according to Mohammad Zahir, head of the Kabul police department's criminal investigation division.

The peace effort suffered a major setback in September 2011 when former Afghan president, Burhanuddin Rabbani, who was head of the peace council, was assassinated by a suicide bomber posing as a peace emissary from the Taliban.

On Twitter, the U.S. Embassy in Kabul called the assassination of another peace council member "a tragedy."

Rahmani, a former member of parliament, was one of about 70 influential Afghans and former Taliban appointed by President Hamid Karzai to try to reconcile with the insurgents.

Rahmani served as minister of higher education during the Taliban regime, which ruled Afghanistan for five years and sheltered al-Qaida before being driven out of power in the U.S.-led invasion in late 2001.

Rahmani was one of several former members of the Taliban who were removed from a U.N. blacklist in July 2011. The decision by a U.N. committee eliminated a travel ban and an assets freeze against Rahmani and the others -- a move seen as key to promoting the peace effort.