The attack, which also wounded another 14 soldiers, targeted an army convoy as it entered jihadist stronghold Zinjibar, capital of Abyan province, said the sources, blaming Al-Qaeda for the bombing.
Forces loyal to President Abedrabbo Mansour Hadi launched the Zinjibar offensive yesterday, after similar assaults pushed the jihadists from other areas in the south.
The Saudi-led coalition that has been battling Iran-backed rebels across the country since March 2015 has deployed Apache helicopters to support the loyalist fighting on ground.
"The withdrawal was decided following information that Al-Qaeda was preparing other car-bomb attacks against our troops," added the officer who requested anonymity.
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Government forces also launched an offensive Saturday to drive the jihadists out of the neighbouring town of Jaar.
Fighting there killed 25 Al-Qaeda fighters and four soldiers as loyalists seized Al-Kud, five kilometres south of Zinjibar, military and medical sources said.
"After our withdrawal, Apache helicopters will target Al-Qaeda positions to secure the town," said another officer, adding helicopters had foiled two attempts to carry out bombings against troops using vehicles in Al-Kud.
Residents reported hearing heavy explosions as coalition jets struck Al-Qaeda-held arms depots in the city.
"The air raids are in preparation for a ground operation as part of a major military offensive to chase Al-Qaeda out of Mukalla and the entire Hadramawt province," an officer said.
Coalition-backed forces have driven militants out of Aden, the southern city declared by Hadi as the country's temporary capital after the Shiite Huthi rebels overran Sanaa in September 2014.
The latest fighting comes as representatives of the government and the Iran-backed rebels continue with UN-sponsored peace talks in Kuwait, which began on Thursday.
The negotiations are under pressure to firm up a fragile ceasefire in their conflict that went into effect on April 11, and from which the jihadists are excluded.