President Abedrabbo Mansour Hadi's chief of staff Ahmed Awad bin Mubarak was handed over to a committee of mediators, 10 days after he was abducted, Hussam al-Shargabi told AFP.
Mubarak was later taken to the Sanaa residence of a tribal chief from Sabwa, the same southern province from which he himself hails.
The Shiite militia, known as Huthis, have controlled most of the Yemeni capital since September 21.
Their seizure of the presidential palace last week prompted Hadi to tender his resignation along with Prime Minister Khalid Bahah, who fled after militiamen encircled his residence.
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The presidential chief of staff was abducted on January 17, apparently in a bid by the militia to extract changes to a draft constitution.
The Huthis had been due to release him immediately under a deal struck six days ago, but they continued to keep him in captivity until today.
Under the deal, they had also pledged to vacate the presidential palace, withdraw from areas surrounding the residences of Hadi and Bahah, and abandon checkpoints across Sanaa, in return for concessions over the draft constitution.
Benomar said Hadi and his cabinet were effectively under house arrest, and warned violence could erupt at any time.
But he added a power-sharing deal "was possible," according to two diplomats who attended a session during which the UN envoy briefed the Security Council by video link from Sanaa.