In this 2017 file photo a Yemeni man looks at a burning vehicle following a reported suicide car bombing in Huta, the capital of the southern province of Lahj, Yemen. AFP
Al-Qaeda in the Arabian Peninsula (AQAP) attacked positions held by the UAE-trained Security Belt group in Abyan governorate in Yemen's south, the sources told AFP.
About three hours of fighting "left 21 dead among the (Security Belt), including an officer, and six among the Al-Qaeda combatants", a government official said on condition of anonymity. Two security sources confirmed the death toll.
Yemen has been gripped by conflict since Iran-backed Huthi rebels took control of the capital Sanaa in 2014, triggering a Saudi-led military intervention in support of the beleaguered government the following year.
AQAP and militants loyal to the Islamic State group have thrived in the chaos.
Tuesday's fighting comes as the Huthis and forces supporting the ousted government observe a shaky ceasefire in the years-long civil war.
Riven by divisions, the groups opposing the Huthis, who originate from the north, include southern separatists who support the re-establishment of South Yemen.
The country was divided into North and South Yemen until reunification in 1990.
Underlining Yemen's parlous security, on Saturday AQAP released a video showing a United Nations worker who was abducted more than six months ago, the SITE Intelligence Group reported.
Five UN staff members were kidnapped in Abyan in February while returning to the port city of Aden after a field mission, UN spokesperson Eri Kaneko told AFP at the time.
In Saturday's video message, apparently recorded on August 9, Akam Sofyol Anam urges "the UN, the international community, the humanitarian organisations, to please come forward... and meet the demands of my captors", without outlining the demands.
Yemen's UN-brokered ceasefire has drastically reduced fighting since the truce began in April, but outbreaks of violence continue.
Last week, 10 Yemeni soldiers were killed in a Huthi attack near Taez, the country's third biggest city which has been blockaded by the rebels since 2015.
The assault, which also left several soldiers wounded, was aimed at cutting off a key route to the southwestern city of about two million, the government said.
On Thursday, the Huthis held a military parade in the Red Sea port city of Hodeida, drawing a rebuke from the UN.