Al-Qaeda militants on Wednesday blew up an army headquarters and set up checkpoints in the Islamist network's southeastern stronghold of Mukalla, officials in Hadramawt province said.
The militants had deployed in force across Mukalla after receiving information of a possible operation by a Saudi-led military coalition to help government loyalists retake the provincial capital, the officials said.
The coalition of Arab states has been carrying out air strikes against Iran-backed rebels in Yemen since March, but has so far not intervened against Al-Qaeda.
Wednesday's explosion flattened the three-storey army building -- the command centre for a zone covering Hadramawt and parts of neighbouring Shabwa province.
It came a day after Al-Qaeda dynamited a headquarters of the secret police in Mukalla, the officials said.
In Aden a 100-strong Saudi force arrived late on Tuesday and was deployed at the main southern city's international airport, military sources said.
"This force's mission is to help secure the city, where Al-Qaeda militants surfaced last weekend," one of the sources said, referring to Aden.
Backed by arms and troops from Saudi Arabia and the United Arab Emirates, government loyalists recaptured Aden from Huthi rebels and their allies in mid-July before retaking four other southern provinces.
But security has remained fragile in the port city, where Al-Qaeda suspects were accused of blowing up a building used by the secret police.
They also set up checkpoints in one district and seized five buildings including an intelligence services facility, a presidential residence and a military hospital, security officials said.
On Tuesday, the International Committee of the Red Cross said it had suspended its operations in Aden after unidentified gunmen stormed its office there.
Meanwhile, Riyadh announced its armed forces on Wednesday intercepted a Scud missile fired from Yemen at the Saudi region of Jazan, adding the explosion caused no casualties or damage.
"The air forces responded immediately by destroying the platform from which the missile was launched" in Yemen, said a statement carried by the official SPA news agency.
The Shiite Huthi rebels' website Sabanews.net had announced that Scud missiles had been fired from Yemen at a power plant in Jazan.
Al-Qaeda in the Arabian Peninsula (AQAP) is considered among the jihadist network's deadliest affiliates.
It has taken advantage of the chaos since the Huthis expanded across Yemen since last year, to seize territory including Mukalla.
AQAP was behind several plots against Western targets, including this year's deadly attack on French magazine Charlie Hebdo.
The United States has for years carried out a drone war against Al-Qaeda militants in Yemen.