AL MUKALLA, Yemen — The Islamic State claimed responsibility for a powerful car bomb that killed the provincial governor in the southern port of Aden early Sunday, raising concerns that hard-line Sunni militants were gaining a foothold in one of Yemen’s largest cities.

The governor, Jaafar Mohamed Saad, was the most senior official to be killed in Aden during a series of assassinations that have signaled the growing strength of Sunni extremists in Yemen’s nine-month civil war. The extremists include militants with Al Qaeda, but more ominously, analysts say, also an increasingly assertive and lethal branch of the transnational Islamic State.

Hours after the blast, in Aden’s Tawahi district, the Islamic State released a statement claiming responsibility, as well as photographs purportedly showing the explosion. The group said it had killed eight of Mr. Saad’s bodyguards.

Aden has been a crucible of fighting in the broader conflict between Houthi rebels from northern Yemen and the government of President Abdu Rabbu Mansour Hadi, which the Houthis drove into exile this year. A Saudi-led military coalition that supports Mr. Hadi began an aerial bombing campaign against the Houthis in March.