A suicide bomber is reported to have killed at least 45 people and wounded more than 40 others during a funeral service in Yemen's southern city of Jaar in Abyan province on Saturday.
Military officials and residents said the bomber targeted tribesmen who sided with the Yemeni army during an offensive against Islamist fighters that the government hailed as a major victory in June.
The military regained control of Jaar from militants in June after two months of fighting.
Officials have opened an investigation to establish a final death toll and to identify the author of Saturday's "criminal and cowardly" attack, Jamal al-Aqal, the province's governor, said in a statement posted on the defence ministry's website.
On Wednesday, an attack by al-Qaeda fighters on a police station in Jaar killed two soldiers and a civilian. Locals have expressed fears that the fighters could retake the town.
Elsewhere, military officials said five suspected al-Qaeda fighters were killed on Saturday in a suspected US aerial drone strike on their vehicle in southeastern Hadramawt province.
Officials spoke on condition of anonymity because they are not authorised to speak to the media.
The US is the only country that has drones in the region and in recent months has been carrying out strikes on al-Qaeda targets in the south and east of the country and in the Arabian Peninsula.
Al-Qaeda has carried out several deadly attacks against Yemeni security services since President Abdrabuh Mansur Hadi, who has repeatedly vowed to battle the fighters, came to power this year.