SANAA (AFP) ― Yemen’s Shiite rebels seized more territory south of the capital Wednesday and set off deadly clashes with al-Qaida as they rapidly expand their area of control.

Already in control of Sanaa since Sept. 21 and the strategic port city of Hudeida since Tuesday, the rebels on Wednesday appeared to have taken control of two more provinces, Dhamar and Ibb, security officials said.

Just as in Sanaa and Hudeida, the rebels faced no opposition as they entered the two provinces and set up checkpoints, the officials said.

The rebels, known as Huthi, have been taking advantage of a power vacuum in Yemen to seize control of significant areas, threatening the authority of the Sunni-led central government, a key ally in U.S. efforts to combat al-Qaida.

Their rapid expansion has raised concerns of Yemen ― already suffering from chronic instability and widespread violence ― spiraling into further unrest.

Deadly fighting broke out Tuesday when the Huthis tried to expand out of the town of Rada in central Baida and clashed with al-Qaida militants, who are active in several provinces.

Five rebels, six suspected al-Qaida militants and a civilian were killed during the fighting in Rada, a security official and tribal sources said Wednesday.

Officially known as Ansarullah, the rebels stretched their control to the shores of the Red Sea on Tuesday, seizing control of Hudeida.

The rebels, traditionally based in the north, have been battling troops and Sunni militants in recent months as part of their bid to spread their control across the country.

Military sources had previously warned that the rebels were looking to take control of Hudeida and to extend their presence to the narrow Bab al-Mandab strait, which leads to the Suez canal.

Bab al-Mandeb, only 40 kilometers across the water from Africa, carried an estimated 3.4 million barrels of oil a day in 2011, according to the U.S. Energy Information Administration.

On Wednesday, groups of rebels advanced on the route linking Hudeida with the commercial seaport of Al-Mukha, just north of Bab al-Mandeb, security sources said.

In the east of the country, the Huthis are apparently eyeing Yemen’s oil and gas reserves in Marib.

Al-Qaida in the Arabian Peninsula, the group’s Yemeni branch, is considered by the United States to be its most dangerous affiliate and its leaders have been targeted repeatedly by U.S. drone strikes.

The group has vowed to fight the Shiite rebels in defense of Yemen’s Sunni majority, and has also repeatedly targeted security forces, particularly in the south and east.

Suspected al-Qaida militants also shot dead an army colonel in southeast Sanaa in other unrest Wednesday, police said.

Four suspected al-Qaida militants, including a local chief, were meanwhile killed in a drone strike in the southern Shabwa province, a tribal source said.

President Abdrabuh Mansur Hadi and the U.S. government have defended the use of drones in Yemen, saying they enable the targeting of jihadists without sending soldiers into lawless areas where the authorities have little or no control.

The impoverished country has been wracked by political turmoil and sporadic violence since an uprising toppled strongman Ali Abdullah Saleh in 2012.