Anti-Houthi demonstrations erupted in Taiz on Sunday, shortly after Houthi militants seized parts of Yemen’s third-largest city. (Facebook/Abdul Aziz Al Samie)

Shiite rebel forces captured new territory in Yemen on Sunday after a call to arms from their leaders, pressing south toward the headquarters of the country’s embattled president and pitching the country further toward sectarian war.

Local officials said Houthi forces had seized the airport in the central city of Taiz, sparking concerns that they were planning a push toward Aden, the port city 120 miles further southwest, where President Abed Rabbo Mansour Hadi has set up a rival governing authority after being driven out of the capital.

After a week of escalating conflict, the U.N. Security Council held an emergency meeting Sunday to address the crisis, which has resulted in the United States withdrawing its remaining military personnel from the impoverished country.

The U.N. special envoy for Yemen, Jamal Benomar, warned at the meeting that events appear to be leading the country “to the edge of civil war” and urged all parties to step back from the brink and resolve the conflict peacefully, the Associated Press reported.

Benomar stressed in a video briefing from Qatar that neither the Houthis nor their opponent, Yemen’s president, could realistically expect to establish control over the whole country.

With rival regional powers of Shiite Iran and Sunni Saudi Arabia backing opposing sides, the struggle has taken on an increasingly sectarian tone, stoking fears that Yemen is becoming the battleground for a proxy war between the two countries.

The instability also has given extremists linked to al-Qaeda an opportunity to step up attacks, with fears that Islamic State militants may also exploit the unrest after an affiliate took credit for the Friday bombing of two Shiite mosques in the country.

“It is difficult to imagine a more dangerous downward spiral than we have seen in Yemen in the last six months,” Rep. Adam B. Schiff (D-Calif.) said in a statement. “The sectarian divide is widening; a geographic chasm has opened between North and South; the proxy battle ground there between Iran and Saudi Arabia has intensified; and al-Qaeda and [the Islamic State] have a new opportunity to grow in this vast ungovernable space.”

The fighters who entered Taiz, both by land and air, were special forces from the Ministry of Interior, which is under Houthi control, said Sheikh Ahmed Mohammed Othman, a prominent anti-Houthi politician in Taiz.

[Read: Who are the Houthis?]

Taiz, Yemen’s third-largest city, is known for its anti-Houthi sentiment, and footage from the city showed large demonstrations near the seized airport Sunday, before tear gas was used to disperse crowds.

“The people of Taiz refuse to have their province used as a starting point for a war against the south,” Othman said. “[The Houthis] want it to be a starting point for their attack on Aden, since it is considered the main gate to the south.”

The push came a day after the Houthi rebel movement called for “a general mobilization” against Hadi’s forces.

“Yemen is definitely closer to hell than it has ever been,” said Farea al-Muslimi, an analyst with the Carnegie Middle East Center, adding that Taiz’s only strategic significance was its position on the road to Aden — and the president’s headquarters.

A security official, who spoke on the condition of anonymity, dismissed theories that Houthis were planning a move south as “rumors” and said that special forces in Taiz were merely providing reinforcements against al-Qaeda.

Houthi leader Abdulmalik al-Houthi said in a televised address that the mobilization of security forces was in order to fight al-Qaeda and the Islamic State, not the people of the south, though he dismissed Hadi as a “puppet” of the extremist groups.

As sectarian rhetoric gears up, the beleaguered president has branded the Houthi actions a coup and described the rebels as proxies of Iran.

Iran weighed also in Sunday, with Deputy Foreign Minister Hossein Amir Abdollahian declaring that the “expectation” is that Hadi should resign “to play a constructive role in preventing the breakup of Yemen and the transformation of Aden into a terrorist haven,” the state news agency IRNA reported.

Former president Ali Abdullah Saleh also has been accused of conspiring with the Houthis against Hadi, with the State Department on Saturday calling on the ousted leader to cease “violent incitement” as it pulled out the last of its troops.

The move has raised concerns that it will undermine counterterrorism operations against al-Qaeda in the Arabian Peninsula (AQAP), the local affiliate of the terror group, which uses Yemen as a staging ground for attacks against the West.

Morris reported from Baghdad.

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