File photo: Armed Houthi rebels ride in the back of a truck during a rally in the capital Sanaa on July 24, 2015. AFP
Twelve others were wounded "in the surprise attack" carried out by the Houthis in the border area between the southern provinces of Lahj and Al-Bayda, the sources told AFP on condition of anonymity.
Four Houthi fighters were also killed and several were wounded, the sources said. There was no immediate comment from the rebels.
The attack targeted a site manned by the separatists, who aspire to create an independent South Yemen such as the state that existed until 1990, the military sources said.
A flare-up of violence has rocked south Yemen in recent months, with several fighters loyal to Yemen's secessionist Southern Transitional Council and soldiers killed in attacks attributed to Al-Qaeda in the Arabian Peninsula.
AQAP is considered by the United States to be the jihadist group's most dangerous offshoot.
Yemen has been gripped by conflict since the Houthis seized the capital Sanaa in 2014, triggering a Saudi-led military intervention in support of the beleaguered government the following year.
Fighting in Yemen calmed markedly after a UN-brokered ceasefire came into effect in April 2022, and has remained largely on hold even after the agreement lapsed last October.
A China-brokered agreement earlier this year that has seen regional power brokers Iran and Saudi Arabia mend ties after a seven-year rupture also sparked hope for Yemen, but peace talks between the warring parties have stalled.
The southern separatist forces are backed by the United Arab Emirates, a key partner in the Saudi-led coalition, and allied with the Yemen government in its fight against the Houthis.
According to the United Nations, the conflict in Yemen has killed hundreds of thousands of people and displaced millions, and two thirds of the population need humanitarian aid.