Unidentified gunmen opened fire and killed a senior officer of Yemen's government forces in the country's southwestern province of Taiz on Tuesday, a government official said.

The official, who asked to remain anonymous, said that Adnan Al-Muhya, an officer of the Criminal Investigation Department (CID), was gunned down by masked assailants in the Al-Jamhouri residential neighborhood.

He said Al-Muhya was leading the investigation into the killing of Moayad Hameidi, director of the United Nations World Food Programme (WFP) office in Taiz. Hameidi was gunned down by unidentified motorcycle-riding gunmen when he was leaving a restaurant in the town of At Turbah, south of Taizin.

Local authorities have not found any suspects or motives behind the killing of the senior CID officer. His death has dealt a blow to the investigation into the murder of the WFP employee, according to the official.

No group has yet claimed responsibility for the attack. Targeted killings of security personnel and government officials have become commonplace as the civil war in Yemen continues for about nine years.

Yemen has been embroiled in a civil war since late 2014, when the Houthi militia attacked several northern cities and forced the internationally-recognized Yemeni government to leave the capital Sanaa.