Military officials and Shiite rebels say Saudi-backed Yemeni troops and resistance fighters failed to take control of Aden's downtown district during intense fighting Saturday.
Military officials say they underestimated the numbers of rebel Houthis fighters, adding that they will regroup and launch another offensive.
Aden, the scene of some of the war's fiercest ground battles, saw Saudi-backed troops and fighters seize from the Houthis some of its neighbourhoods and its international airport last week.
Yemen's conflict pits the Iran-allied Houthis and troops loyal to former President Ali Abdullah Saleh against an array of forces, including southern separatists, local and tribal militias, Sunni Islamic militants as well as loyalists of exiled President Abed Rabbo Mansour Hadi, who is backed internationally.