Colonel Khaled al-Rida was killed in clashes in the rebel-held capital Sanaa late Saturday, a statement released by Saleh's General People's Congress party said.
Rida was the deputy head of foreign relations in the party.
While the statement did not name the Huthi rebels, it accused a "group that knows no morality or oaths" of being behind the killing -- a thinly-veiled reference to the Huthis, who over the past week have lashed out at Saleh as a "back- stabber" and "traitor" as tensions rise within the rebel alliance.
A source within the GPC said the clashes erupted at a rebel checkpoint in the southern neighbourhood of Hadda after a dispute between fighters manning the checkpoint and armed supporters of Saleh who were driving by.
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Saleh and his one-time foe Abdul Malik al-Huthi joined ranks in 2014 in a shock alliance that drove the internationally recognised government out of Sanaa and into the southern province of Aden.
A war of words between Saleh and rebel leader Huthi has escalated in the past week.
The Huthis reportedly suspect Saleh has been negotiating with a Saudi-led military coalition that supports the Yemeni government.
Saleh, meanwhile, is said to be displeased with the Huthis' newfound power in the capital, where they run a number of key offices.
The war between the government of President Abedrabbo Mansour Hadi, backed by the coalition, and the rebel camp has killed more than 8,300 Yemenis since 2015 and pushed the country to the brink of famine.