ADEN, Yemen — Yemen’s Shiite rebels issued a call to arms Saturday to battle forces loyal to the country’s embattled president, as U.S. troops were evacuating a southern air base crucial to America’s drone strike program after al-Qaida militants seized a nearby city.

The turmoil comes as Yemen battles al-Qaida in the Arabian Peninsula, the target of the drone program, and faces a purported affiliate of the extremist Islamic State group that claimed responsibility for a series of suicide bombings killing at least 137 people Friday.

All these factors could push the Arab world’s most impoverished country, united only in the 1990s, back toward civil war.

“I hate to say this, but I’m hearing the loud and clear beating of the drums of war in Yemen,” Mohammed al-Basha, a spokesman for the Yemeni Embassy in Washington, D.C., wrote on Twitter.

The Shiite rebels, known as Houthis, swept into Yemen’s capital, Sanaa, in September and now control it and nine of the country’s 21 provinces.

President Abed Rabbo Mansour Hadi, a one-time prisoner of the Houthis in his own home, escaped last month and installed himself in Aden, declaring it the temporary capital amid the Houthi insurrection.

Earlier Saturday, Hadi gave his first televised address since fleeing the capital, striking a defiant tone. He described the rebels’ rule as “a coup against constitutional legitimacy.” He also pledged to raise the Yemeni flag over the Maran mountains, a stronghold for the Houthis, members of the Shiite Zaydi sect that represents nearly 30 percent of Yemen’s population.

Hadi also said regional Shiite power Iran supported the Houthis, something critics also allege and the rebels deny.

Almost immediately after Hadi’s speech, the Houthis issued a statement announcing their offensive against security and military institutions loyal to Hadi, calling it a battle against extremists.

Meanwhile Saturday, U.S. troops including Special Forces commandos were evacuating from the al-Annad air base in southern Yemen, Yemeni security and military officials said. The officials did not say whether the troops had left the country.

The air base, the country’s largest, was believed to have some 100 American troops stationed there. U.S. officials did not immediately respond to a request for comment Saturday.

Saturday night, a security official in Aden said a military transport plane from Oman evacuated 16 British military and security forces.

On Friday, al-Qaida militants seized control of the southern provincial capital of al-Houta in the group’s most dramatic grab of territory in years. That’s just nearby the al-Annad air base, which has been the scene of rocket attacks in the past by militants.

Maj. Gen. Mahmoud al-Subeihi, the country’s defense minister who is loyal to Hadi, said troops would be deployed near the base to protect it from militants.

The al-Annad base is where American and European military advisers help Yemen battle the country’s local al-Qaida branch through drone strikes and logistical support. That group, which holds territory in eastern Yemen, has said it directed the recent attack against the French satirical magazine Charlie Hebdo in Paris.

All this comes a day after suicide bombers attacked a pair of mosques in Sanaa, unleashing monstrous blasts that killed 137 people, including at least 13 children. A purported affiliate of the Islamic State group claimed responsibility for the bombings, which also wounded 357 people – raising the alarming possibility the extremist group has expanded its presence to Yemen after already setting up a branch in Libya.