Updated at 5:20 a.m. ET: KABUL - Gunmen shot dead a top Afghan peace negotiator in the capital Kabul on Sunday, police said, dealing a massive blow to the country's attempts to negotiate a peace deal with Taliban insurgents. 

Maulvi Arsala Rahmani was one of the most senior members on Afghanistan's High Peace Council, set up by President Hamid Karzai two years ago to open talks with insurgents. 

"He (Rahmani) was stuck in heavy traffic when another car beside him opened fire," said General Mohammad Zahir, head of the investigations unit for Kabul police. 

Massoud Hossaini / AFP - Getty Images

Arsalan Rahmani's death on Sunday was seen as a major blow to Karzai's U.S.-backed peace efforts.

Rahmani, one of about 70 influential Afghans and former Taliban appointed by Karzai to try to reconcile with the insurgents, was on his way to a meeting with lawmakers and other officials in a government-run media center in the heavily barricaded diplomatic center of Kabul. 

"His driver did not immediately realise that Rahmani had been killed," police official Zahir told Reuters, adding that no one had been arrested in connection with the shooting. 

No group took responsibility for the attack. 

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Rahmani was essential to the peace process, a BBC News producer in Kabul quoted an Afghan legislator as saying.  

"(The) killing of Rahmani is the work of those who are against Peace Process. Rahmani was the backbone of (the) peace process," BBC News producer Bilal Sarwary quoted MP Mirwais Yasini as saying in a tweet.

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.  He 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.

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NATO in Afghanistan condenmed the killing.

"The only possible aim of this attack is to intimidate those, who like Rahmani, want to help make Afghanistan a better place for its citizens and the region," it said in a statement. "This attack is clear evidence that those who oppose the legitimate government of Afghanistan have absolutely no interest in supporting the peace process on any level but through murder, thuggery, and intimidation."

The head of the peace council and former Afghan president, Burhanuddin Rabbani, was assassinated by a suicide bomber last year. 

In separate news, U.K. officials announced that two British servicemen were shot by members of the Afghan police force

The Ministry of Defence said the two had been providing security near a base in the Lashkar Gah in Helmand province.

Reuters, The Associated Press and msnbc.com staff contributed to this report. 

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