BEIRUT - Shiite insurgents called on supporters to attack institutions under the control of Yemen's embattled president on Saturday amid reports that 100 U.S. troops were preparing to evacuate a base in the southern part of the country after al-Qaeda briefly captured a neighboring city.

The call to arms by the insurgents, known as the Houthis, appears to have pushed the desperately poor Arabian Peninsula country into a state of civil war after months of mounting violence. It also comes as the widening chaos - which included suicide attacks in the capital, Sana'a, on Friday that killed scores of people - has undermined U.S. counterterrorism operations against al-Qaeda in the Arabian Peninsula, or AQAP, which uses Yemen as a staging ground for attacks against the West.

Published by the state-run news agency, Saba, the Houthi declaration calls for a "general mobilization" against the forces loyal to President Abed Rabbo Mansour Hadi. Last month, he established a rival governing authority in the southern city of Aden after escaping captivity by Houthi militiamen in Sana'a, which they control.

The Houthi declaration was issued shortly after Hadi gave televised address, his first since fleeing to Aden, stating that he was the legitimate leader of Yemen and calling the Houthi actions a "coup." In the address, he demanded that the militants withdraw from government buildings, return looted weapons and participate in peace talks that have been proposed to be held in Saudi Arabia.

In a sign that Yemen's violence is taking on a sectarian dimension, Hadi also referred to the Houthis as proxies of Iran. "We shall deliver the country to safety and raise Yemen's flag on Mount Maran in Saada, instead of the Iranian flag," he said, referring to a northern province that is the Houthi stronghold.

Shiite Iran has boosted support for the Houthis, who follow an offshoot of Shiite Islam known as Zaydism. Hadi's authority, meanwhile, has received increasing support from Iran's primary nemesis, Saudi Arabia, which is Sunni, raising concern among Yemenis that their country is becoming a proxy battleground between the regional powers.

On Thursday, Hadi's allies blamed the Houthis for launching an airstrike that attempted to hit, but missed, the president's palace in Aden. That alleged attack followed hours of intense clashes in the city between Hadi's forces and military units loyal to Ali Abdullah Saleh, who was ousted by nationwide protests in 2011. Many Yemenis accuse Saleh of conspiring with the Houthis against Hadi.

Hadi's address came a day after suicide bombers targeted two Houthi-linked mosques in Sana'a killed as many as 137 people, in what was one of the deadliest attacks in the country since the civil war of 1994.

A group alleging to be a wing of the Islamic State claimed responsibility for Friday's bombings. The Sunni-militant group that has sowed chaos in Iraq and Syria is not known to have a presence in Yemen, but the incident highlights how extremists are exploiting the unrest that began when Houthis insurgents captured Sana'a in September and toppled Hadi's government in January.

The escalating violence appears to have convinced the U.S. to withdraw its remaining security presence in Yemen. Some 100 special forces troops, who specialized in assisting Yemeni forces in the fight against AQAP, were leaving the al-Annad air base, CNN reported. It is unclear whether the forces have departed the facility, which is in southern Yemen.

On Friday, militants from AQAP captured a city that is close to the base before being repelled by local military units. Located in the Lahj province, the city, called al-Houta, was briefly taken by the Sunni militants in fierce clashes that killed as many as two-dozen Yemeni soldiers.

AQAP has been targeted by U.S. drone strikes for years in coordination with Yemeni forces, but the unrest has jeopardized that counter-terrorism coordination. The U.S. withdrew its embassy personnel from Sana'a in February, and much of the Yemeni intelligence and security apparatus that participated in coordinating drone strikes has fallen under the control of the Houthis.

In February, AQAP militants stormed a military base in the southern Shabwa province, using sophisticated attacks involving car bombs. They were forced to leave by local tribesmen. AQAP also said that it ordered the January attack in Paris again the French satirical magazine Charlie Hebdo, although the group denied involvement in Friday's suicide bombings in Sana'a.

Meanwhile, concern is rising that Houthi fighters were preparing assaults in other areas of the country. There were unconfirmed reports on Twitter of another Houthi attack on Saturday in the oil-rich province of Marib. The group mounted an assault on the province last week. Capturing Marib, about 75 miles east of the capital, would give the insurgent group access to oil and gas facilities as well as power plants that supply the capital and other areas of the country with electricity.

There are also unconfirmed reports of an attempted assault by the Shiite rebels on the city of Taiz, in central Yemen, where anti-Houthi sentiment runs high.