Islamic State claimed responsibility for an attack on the army in the eastern region of Hadramawt on Friday that a security source said killed at least 19 Yemeni soldiers and 35 militants.
Earlier on Friday a different security source said Al-Qaeda in the Arabian Peninsula (AQAP) was responsible for the attack.
Islamic State's Yemen branch has attacked both main sides in the country's civil war in recent months, targeting the Shi'ite Houthi militia in mosques in the capital Sanaa as well as Saudi-led forces and a local grouping of anti-Houthi fighters, with suicide blasts in Aden in September that killed dozens.
Islamic State said in a statement it had killed nearly 50 soldiers in the attack, many more than the number cited by local officials, and just one of its fighters was killed carrying out a suicide bombing using a car in the assault.
Fighting was still going on in the area after the initial attack by militants between the towns of Shibam and al-Qatn, the security source said. Islamic State said it had targeted three separate army posts.
Unverified footage on social media purporting to show the attack included a large blast followed by a big plume of smoke, and the sound of shooting as well as distant voices shouting.
Bombing and shooting attacks were commonly directed against the army and police across Hadramawt from 2011 until the civil war intensified early this year with the start of a military campaign by a Saudi-led Gulf alliance.
Although AQAP has controlled the southern Hadramawt city of Mukalla and surrounding areas most of this year, it has not attacked army positions elsewhere in the province for months.
The conflict has split Yemen's army. Many soldiers in Houthi-controlled areas near Sanaa are loyal to former president Ali Abdullah Saleh, who is allied to the Houthis.
However in Hadramawt, hundreds of miles (km) east of the main battlefronts that are separated by a broad stretch of desert, the army is loyal to President Abd-Rabbu Mansour Hadi, whose government Saudi Arabia wants to restore.
While Islamic State regards the Houthis as heretics because they are Shi'ites, it also views Yemen's army, and its Gulf backers as traitors to Islam. Its attacks in Yemen appear aimed at stoking instability to grow its support base, analysts say.
