The power struggle between Yemen’s internationally-recognized president, Abd Rabbuh Mansur al-Hadi, and Shia rebels from the north of the country worsened on Thursday with clashes in the port city of Aden reportedly killing thirteen.

Hadi fled to Aden after the Houthis dissolved parliament in Sana’a last month. They first stormed the capital city in September and had held Hadi under house arrest.

Forces loyal to Hadi fought their way into Aden’s airport on Thursday and wrestled a nearby military base from a renegade officer, officials said. An unidentified warplane attacked Hadi’s residence.

It was unclear if the forces opposing Hadi were Houthis or supporters of his predecessor, Ali Abdullah Saleh.

In a statement, Hadi lashed out at both, saying the Houthis had staged a “military coup” and accusing “the former regime” of perpetuating “extermination.”

Earlier this month, several people were killed in confrontations between Hadi supporters and Saleh loyalists in Aden.

Yemen’s Arab Gulf neighbors forced Saleh to step down in 2012 after more than two decades in power, fearing an “Arab Spring”-style uprising in the country. The same monarchies now support Hadi who is also backed by Western powers. Saleh remains in charge of Yemen’s ruling political party.

Hadi’s supporters call the Houthis “agents” of Iran and the Shia state has rushed to help the rebels consolidate their position in Sana’a. A delegation of Houthi leaders traveled to Tehran earlier this month. Iranian medical supplies arrived in Yemen a day after the two regimes had signed an aviation agreement.

Hadi still commands the loyalty of local authorities in the Hadhramaut, Ma’rib and Shabwah provinces which contain most of Yemen’s oil and natural gas reserves. But he has failed to win over factions that advocate the secession of the former South Yemen as well as members of Saleh’s clan. Their refusal to support him allowed the Houthis to take over Sana’a in the first place.

Most of the former North Yemen is controlled by the Houthis while large swaths of the southeast are effectively in the hands of Al Qaeda terrorists.

The two Yemens were unified in 1990 under Saleh’s leadership. The south seceded four years later, triggering a civil war and northern occupation in 1994.