SANAA, Yemen (AP) — Fierce battles between Shiite Houthi rebels and pro-government forces in Yemen's Marib province and the city of Taiz Friday killed 20 people, according to officials.

Nine rebels and seven anti-Houthi tribesmen were killed in clashes in Marib, medical officials said. Houthi shelling killed four civilians in the city of Taiz, according to medical and security officials.

All officials spoke on condition of anonymity because they were not authorized to talk to reporters.

The Saudi-led coalition backed pro-government fighters in Friday's clashes, bombing rebel positions.

Clashes also raged north of the strategic southern port city of Aden, according to a security official. Anti-Houthi fighters in Aden said Thursday they seized full control of the city, after they pushed the rebels from their last holdout.

This current push north of Aden is the start of a wide-scale campaign to clear the city's surrounding area of rebels, a security official said. The anti-Houthi forces made significant gains in Aden over the past week, with battles to drive the rebels and their allies out of the city stretching on for days.

The battle for Aden has become the epicenter of Yemen's conflict over the past several weeks. The Iran-allied rebels seized the Yemeni capital, Sanaa, last September and then began pressing their advance south. Fierce fighting over Aden broke out in March, sparking the Saudi-led airstrikes that have been targeting the rebels to halt their advance.

The fighting in Yemen pits the Houthis and troops loyal to former President Ali Abdullah Saleh against southern separatists, local and tribal militias, Sunni Islamic militants and loyalists of exiled President Abed Rabbo Mansour Hadi, who is now in Saudi Arabia.