Unidentified gunmen shot dead a law professor who has been representing Shiite Huthi rebels at Yemen's thorny reconciliation talks which are due to end on Tuesday, a security official said.
The killing came as representatives to the national dialogue convened for a final working session, during which President Abrabuh Mansur Hadi set Saturday as a date for a ceremonial conclusion.
Ahmed Sharafeddin, a leading figure in the rebel group, was shot dead as he drove from his house in the capital Sanaa to the hotel where the talks are being held, the official said.
He was attacked at the Saba roundabout in central Sanaa and died instantly, the official added, speaking on condition of anonymity.
Sharafeddin is the second Huthi representative to the talks to be murdered, after MP Abdulkarim Jadban was shot dead in a similar attack in November.
The killing comes as fighting between Huthis and powerful tribes resumed in Amran province after a series of ceasefires brokered by the government halted months of battles in several northern areas.
The rebels, whose stronghold is the northern province of Saada, accuse the tribes of backing hardline Sunni Salafists fighting the Huthis in the enclave of Dammaj.
The town in Saada province has been besieged by the rebels for months.
Huthi rebels have been battling the Sanaa government for nearly a decade in Saada, but the clashes with Sunni militants have deepened the sectarian dimension of the unrest.
Fighting that erupted in late October has centred on a Salafist mosque and Koranic school in Dammaj.
The conflict has spread in the northern provinces, embroiling Sunni tribes wary of the power of the Huthis, who have repeatedly been accused of receiving support from Iran.
Yemen's national dialogue is part of a transition backed by the United Nations and the Gulf countries which saw President Ali Abdullah Saleh step down after 33 years in power following massive Arab Spring-inspired protests in the region's poorest country.
It aims to draft a new constitution and prepare for general and presidential elections to replace interim president Hadi.
"The dialogue will continue, and evil forces will fail," Hadi told participants as some Huthi members walked out following news of Sharafeddin's killing.
The UN representative to Yemen, Jamal Benomar, condemned the killing, slamming it in a statement as a "desperate attempt to jeopardise the dialogue".