Suspected Al-Qaeda fighters killed at least 54 soldiers and police in a wave of dawn attacks Friday, the deadliest day for Yemeni security forces since jihadist strongholds fell last year.
The militant assaults came in the lawless southern province of Shabwa, a bastion of Al-Qaeda in the Arabian Peninsula (AQAP) and the scene of regular US drone attacks targeting the militants.
Military officials said there were four attacks in all, including one on a key gas export terminal that was thwarted.
Al-Qaeda's offensive came a month after officials said militant plans to attack oil and gas terminals had been scuppered following intelligence eavesdropping on a call from the organisation's overall chief, Ayman al-Zawahiri.
Friday's deadliest single attack was on an army camp responsible for ensuring security at oilfields in Shabwa, and killed 38 soldiers, sources said.
"Troops clashed with gunmen at the camp entrance, before a suicide attacker in a bomb-laden vehicle forced his way into the camp where his car exploded, killing 38 soldiers," said a government official in Ataq, the Shabwa provincial capital.
Military sources confirmed the toll.
Simultaneously, "a suicide bomber in a car blew himself up before reaching his target -- an army checkpoint" in the nearby Al-Nushaima area, a military official said, adding that 10 soldiers were killed in that blast.
"Soldiers were captured" in Al-Nushaima as others fled, witnesses told AFP by phone.
Around 15 kilometres (nine miles) away, suspected Al-Qaeda gunmen targeted a special forces camp at Maifaa, killing eight policemen, military sources said.
At least eight militants, among them two suicide bombers, were reported killed in the three attacks.
The defence ministry in Sanaa said a fourth Al-Qaeda attempt to detonate explosives targeting the Ain Ba'maabad region and Balhaf gas terminal ended in failure.
Security Forces intercepted the vehicle which exploded, "killing the terrorists it was carrying," said the ministry's 26sep.net news website, without stating how many militants died.
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The authorities blamed Friday's bloody battles on AQAP, described by Washington as the jihadist network's deadliest franchise.
Security force losses were the deadliest since May 21, 2012 when some 100 soldiers were killed and hundreds more were wounded in a suicide bombing in the capital.
AQAP claimed that attack in a statement posted on jihadist Internet forums.
The following month, the army recaptured large swathes of the south which Al-Qaeda had held for nearly a year.
AQAP had taken advantage of the weakness of the central authority during the 2011 uprising that forced out veteran president Ali Abdullah Saleh.
Since then, the extremist group has launched mainly hit-and-run attacks, with its members under the constant threat of monitoring and missile attack from unmanned US aircraft.
President Abdrabuh Mansur Hadi said that a wave of drone strikes on Al-Qaeda targets in August killed 40 militants in two weeks, including some ringleaders in the Sanaa region.
On Sunday, a court in the capital jailed three AQAP militants for plotting to assassinate Hadi and the US ambassador.
And last month, security was beefed up around Western embassies in Sanaa, and some were closed following warnings by Washington of an imminent attack.
Since succeeding Saleh, Hadi has repeatedly pledged to press the battle against Al-Qaeda in what is Osama bin Laden's ancestral homeland.
Hadi said in August a bid to attack the Balhaf terminal had been foiled after a phone call was intercepted between Zawahiri and AQAP leader Nasser al-Wuhayshi.
Sanaa also said it had foiled an Al-Qaeda plot to storm the Canadian-run Mina al-Dhaba oil terminal and seize the port of Al-Mukalla, capital of the eastern province of Hadramawt.
AQAP has denied plotting any such attacks.