Sana’a and Riyadh, Asharq Al-Awsat—Fighters from the Shi’ite Houthi movement seized Yemen’s third-largest city Taiz late on Saturday as the political crisis currently gripping the country threatened to boil over into a full-scale internal conflict.

Local eyewitnesses told the Reuters news agency armed Houthi rebels took control of Taiz’s airport on Saturday evening. This followed an earlier announcement on the same day by the Houthis’ “Revolutionary Council”—set up to oversee the country’s affairs following the group’s power-grab in February—that it was mobilizing the Yemeni armed forces due to what it said was the current “state of war” in the country.

Informed sources from Yemen’s capital Sana’a, also currently controlled by the Houthis, told Asharq Al-Awsat that military transport aircraft had airlifted “hundreds of armed Houthis” to a military base in Taiz late on Saturday.

They added that members of the Yemeni Special Forces were also heading to Taiz, traveling via the Taiz–Aden highway, and had clashed with civilians en route to the city.

Sources have said that elements within the country’s army remain loyal to former president Ali Abdullah Saleh, who along with Iran has aided the Houthi coup and the movement’s advance across parts of the country.

Tribal sources from Ma’rib, the country’s main oil and gas hub, told Asharq Al-Awsat on Saturday the Houthis had staged a number of military patrols outside the province and clashed with armed local tribal groups. Twenty members of the movement were killed during the clashes, the sources said.

This comes as President Abd Rabbuh Mansur Hadi gave his first official address to the nation since heading to the southern port city of Aden in February after escaping a Houthi-imposed house arrest.

Hadi, who is seeking to establish a rival power-base to the Houthis’ in Sana’a, said he would continue his efforts until the flag of the Yemeni Republic was hoisted over the northern city of Saada, the Houthis’ traditional stronghold.

He demanded the Houthis’ withdraw from all government buildings they have occupied, return all weapons seized from military facilities back to the state, and comply with UN Security Council and Gulf Initiative directives.

He also called on the Houthis’ to abide by an agreement signed with Hadi following the movement’s takeover in Sana’a, which called on the group to withdraw from the capital and other parts of the country.

Hadi also announced during the address that he had appointed a new foreign minister, Riyadh Yassin, in place of Abdullah Al-Saidi, who like Hadi and Prime Minister Khaled Bahah was placed under house arrest by the group. Bahah, Saidi and other members of Hadi’s cabinet placed under house arrest were freed by the Houthis last Monday.

Speaking to Asharq Al-Awsat, new Foreign Minister Yassin called on the Houthis to attend an upcoming conference in Riyadh in April which aims to resolve the crisis in the country.

“If the Houthis truly have a cause, then they must put their cards on the table and speak openly with us in front of everyone . . . without seeking to control things or launching airstrikes on Aden or harming civilians. This is unacceptable,” Yassin said.

On Thursday an unidentified warplane attacked the presidential palace in Aden where Hadi is currently residing, though he was not present at the compound at the time.

On the same say, two suicide bomb attacks also targeted worshipers at Houthi-controlled mosques in Sana’a, leaving 140 people dead.

Violent clashes between armed Houthi rebels and armed civilians and tribesmen loyal to Hadi also continued in Aden and Sana’a on Saturday.

Nasser Al-Haqbani contributed additional reporting from Riyadh.