(Adds context, background)
SANAA, Jan 21 (Reuters) - Unknown assailants shot dead a leader of the Yemeni Shi'ite Muslim Houthi group in Sanaa on Tuesday while driving to attend the final session of reconciliation talks aimed at tackling the country's political and security challenges.
Yemen has been torn by rising violence and lawlessness as the U.S.-allied country struggles to overcome political turmoil after long-serving President Ali Abdullah Saleh stepped down following months of mass protests against his rule in 2011.
Officials said that Ahmad Sharafeddin, a Houthi delegate at the reconciliation talks who had been dean of the law faculty at Sanaa University, was killed when gunmen opened fire on him in his vehicle from a speeding car in central Sanaa.
They said he died instantly and the gunmen escaped.
It was not immediately clear who was behind the assassination, but another Houthi leader, Abdulkarim al-Khiwani, accused hardline Sunni Muslim militants of carrying out the attack.
The Houthi group fought radical Sunni Salafis in northern Yemen from October until earlier this month, when a ceasefire was reached earlier to relocate the Salafis to another city some 250 km (155 miles) away. But clashes have continued in other parts of northern Yemen with tribesmen allied to the Salafis.
More than 210 people have been killed in the fighting that erupted in late October after the Houthis accused the Salafis of recruiting foreign militants in preparation to attack them.
The Salafis, who follow an austere brand of Sunni Islam, say the foreigners are students of Islamic theology.
The sectarian rivalry has cast a shadow over reconciliation efforts in Yemen, a neighbour of top oil exporter Saudi Arabia and home to one of al Qaeda's most active wings.
Tuesday's attack was the latest in a string of killings against high-profile Yemenis and foreigners. Last week an Iranian diplomat was killed in Sanaa when he resisted gunmen who were trying to kidnap him.
Despite the challenging security conditions, President Abd-Rabbu Mansour Hadi addressed the final session of the reconciliation talks, which is due to produce a new constitution for Yemen and approve a federal system for the country.
The talks, which began in March last year, have been stuck on southern separatist demands to revive the state that merged with North Yemen in 1990.
The delegates have agreed on a federal system but have yet to work out some of the details, which will be tackled by a special committee headed by Hadi.
(Reporting by Mohammed Ghobari, writing by Sami Aboudi; editing by Yara Bayoumy and Tom Heneghan)