By Mohammad Mukhashaf

ADEN, Yemen (Reuters) - Al Qaeda-linked fighters seized a large army base in a dawn attack in southern Yemen on Thursday, militants and residents said, hours after the United Nations warned that the country was on the brink of civil war.

The base in the southern province of Shabwa, housing a brigade of up to 2,000 government soldiers, fell after several hours of heavy clashes, residents and local news sites said.

The al Qaeda-affiliated Ansar al-Sharia said on Twitter it had set off a suicide bomb at the gate and imprisoned some of the troops.

Al Qaeda and other Sunni Muslim militants have stepped up attacks since rival Iranian-backed Shi'ite Muslim fighters from the north seized the capital in September and started expanding across the country.

The Houthis have sidelined the central government and have clashed with Sunni tribesmen in Yemen, which borders oil giant Saudi Arabia.

"We believe the situation is very dangerous. Yemen is on the brink of civil war," Jamal Benomar, the U.N. Special Envoy to Yemen, said in an interview with television channels al Arabiya and al-Hadath late on Wednesday.

He accused all sides of contributing to the political and economic turmoil and called for more talks.

An official from the town of Beyhan near the seized army base said residents feared the Houthis would now move in to confront the Ansar al Sharia fighters.

"We are scared this (the capture of the army base) is going to be used as a justification for a Houthi attack and that they will take over Shabwa with the help of the army," the official said, refusing to be identified.

Ansar al-Sharia said after bombing the entrance of the base, it took control of three guard towers and one tank.

"By approximately eight o'clock in the morning, the mujahideen had imprisoned most of the soldiers inside," the group said on Twitter.

Yemen's government was a key ally of Washington in its war on al Qaeda. The United States has been carrying out drone strikes on militant targets for over a decade, many in the south.

But the United States, as well as Britain and France, closed their embassies in the capital Sanaa on Wednesday, citing security concerns since the Houthi takeover.

Yemen is also home to Al Qaeda in the Arabian Peninsula, one of the global network's most active arms that has carried out attacks abroad.

(Additional reporting by Mohammed Ghobari in Sanaa and Omar Fahmy in Cairo; Writing by Raissa Kasolowsky; Editing by Ralph Boulton and Andrew Heavens)