Shiite rebels fired bullets and tear gas to disperse thousands of protesters demanding they withdraw from a southern province, killing six demonstrators, wounding scores more and escalating tensions in a country on the verge of civil war.

The rebels, known as Houthis, seized the capital Sanaa in September and have been advancing south alongside forces loyal to former President Ali Abdullah Saleh. In recent days they have closed in on the southern port city of Aden, where the internationally recognized President Abed Rabbo Mansour Hadi is now based.

Massive protests were held in the third largest city of Taiz -- which the rebels largely seized over the weekend -- and in Torba, some 60 miles (100 kilometers) away, where witnesses said the streets were filled with thick black smoke from burning tires and where protesters torched three armored vehicles.

"Torba turned into a ball of fire," said Khaled al-Asswadi, a resident. He said the protesters managed to prevent the Houthis from advancing into the city.

A medical official said six protesters were killed and dozens wounded in Torba. Local activists posted pictures on social media of what they said were dead protesters, their clothes drenched in blood.

Another witness, Mohammed Salem, said that Houthis and Saleh's forces fired anti-aircraft guns to scare off the protesters, "but the number of protesters increased instead." In a statement, Yemen's Socialist Party warned that the Houthis' invasion of the mostly Sunni south would set off a "sectarian war."

Taiz is Yemen's third largest city and the birthplace of the Arab Spring-inspired uprising in 2011, which forced Saleh to hand over power to Hadi in a deal brokered by the U.N. and Gulf countries the following year.

But Saleh never fully retired, and has been widely accused of acting through his loyalists in the government and security forces to derail the country's democratic transition. He is now allied with the rebels, and his loyalists helped the Houthis to take over the airport and other government buildings in Taiz.

In addition to dispersing the protesters, the Houthis also engaged in heavy fighting with militias loyal to Hadi in the city of al-Dhalea, where the two sides used artillery, anti-aircraft guns and machine guns, according to a Yemeni security official. The Houthis and troops loyal to Saleh have taken over the governor's office there.

All officials spoke on condition of anonymity because they were not authorized to speak to the press.

The Houthis hail from the Shiite Zaydi community, which makes up around a third of Yemen's population and is concentrated in the north. The Houthis' opponents view them as a proxy of Shiite Iran, charges they deny.

Hadi fled house arrest in Sanaa last month and has set up a base in Aden, the capital of the once-independent south. On Monday he called on Gulf countries to intervene against the rebels and for the U.N. to set up a no-fly zone.

Saudi Arabia's Foreign Minister Saud al-Faisal meanwhile warned that "if the Houthi coup does not end peacefully, we will take the necessary measures for this crisis to protect the region."

The six-nation Gulf Cooperation Council -- Saudi Arabia, Kuwait, Qatar, the United Arab Emirates, Oman and Bahrain -- warned earlier this year that they would act to protect the Arabian Peninsula's security and described the Houthi takeover as a "terrorist" act.