The attack is the latest in a series of deadly operations against security forces blamed on al Qaeda in the Arabian Peninsula (AQAP), perceived by Washington as the militant network's most dangerous affiliate.
The gunmen "suspected of belonging to al Qaeda" opened fire on the checkpoint at the western entrance to the port city of Mukalla, the provincial capital of Hadramawt, the official said.
Two other soldiers were wounded, the official said, requesting anonymity.
The assailants also stormed a nearby police station where they stripped officers of their weapons but spared their lives, he said.
On Friday, suspected al Qaeda gunmen killed eight soldiers in an attack on an army post in Hadramawt. Four militants died in the attack.
It came after the group claimed a brazen assault Wednesday on an army headquarters in a highly secure area of Aden, which an official said killed six soldiers and three civilians, in addition to 10 assailants and a suicide bomber.
A security official gave a death toll of six soldiers, three civilians, 10 assailants and a suicide bomber from that attack.
AQAP said the attack killed "nearly 50 soldiers" and was part of its campaign to "target the joint operation rooms that manage the US drones in the country".
US drones frequently attack militants on the run in the rugged terrain of the south Arabian Peninsula country.
COMMENTS
Comments are moderated and generally will be posted if they are on-topic and not abusive.
For more information, please see our Comments FAQ