Suspected al Qaeda fighters killed at least 56 soldiers and police in a wave of dawn attacks on 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 militants.
Military and government officials said there were four attacks in all, including one on a key gas export terminal that was foiled.
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.
Thirty-eight soldiers were killed in Friday's deadliest single attack, on an army camp responsible for ensuring security at Shabwa oilfields, the 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 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, 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, according to military sources.
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 Balhaf gas terminal on the Gulf of Aden 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 specifying how many militants died.
The authorities blamed Friday's bloodshed on AQAP, described by Washington as the jihadist network's deadliest franchise.
Losses among the security forces were the deadliest since May 21, 2012 when some 100 soldiers were killed and hundreds more wounded in a suicide bombing in the capital.
AQAP claimed that attack in a statement posted on jihadist Internet forums.
AQAP 'exploiting political crisis'
In June 2012, the army recaptured large swathes of the south which al Qaeda had held for nearly a year.
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.
According to Zaid al-Salami, a Yemeni specialist on Islamist groups, al Qaeda "has exploited the political crisis in Sanaa and differences over the results of the national dialogue" to strengthen its presence in Shabwa.
The reconciliation talks, which had been scheduled to end this week, were extended by around one month after participants failed to reach agreement on the future shape of the state.
A special committee had been close to signing a roadmap to solve the issue of the formerly independent south Yemen this week.
But the signing was put off after the two representatives of former president Ali Abdullah Saleh's General People's Congress walked out and the GPC suspended its participation in the committee.
Critics accuse Saleh's loyalist of trying to hamper a transitional process stipulated by a UN-backed initiative which ended a year of protests and ousted him in February 2012, after 33 years as president.
AQAP is also "sending a message to the Yemeni government and the United States that drone strikes have not limited its activities and presence on the ground," said Salami.
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 its slain leader 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.