The men were at a staging area near two schools and a mosque where they were registering to join the expedition. The Saudis hope to train up to 5,000 fighters and deploy them to the Saudi cities of Najran and Jizan, near the border, Yemeni security officials said. Over 60 wounded were being taken to three area hospitals, they added.
Aid group Doctors Without Borders reported on social media that their hospital in Aden had received 45 dead, while the Yemeni officials earlier put the figure at 25 but said it was likely to rise. The officials spoke on condition of anonymity because they weren't authorized to brief journalists, AP reported.
The Daesh-run Aamaq news agency said the attack was carried out "by a fighter from Daesh who targeted a recruitment center."
Yemen is embroiled in a civil war pitting pro-Hadi forces and a Saudi-led coalition against the Houthis, who are allied with army units loyal to a former president. The fighting has allowed al-Qaeda and a Daesh affiliate to expand their reach, particularly in the south.
The UN and rights groups estimate at least 9,000 people have been killed since fighting escalated in March 2015 with the start of Saudi-led airstrikes targeting the Houthis and their allies. Some 3 million people have been displaced inside the country, the Arab world's poorest.
UN-mediated peace talks in Kuwait were suspended earlier this month with no signs of progress.
The Houthis and forces allied to former President Ali Abdullah Saleh seized Yemen's capital, Sana’a, in September 2014, forcing the Hadi government to flee the country. The Saudi-led campaign against the Houthis has pushed them out of southern Yemen, but has failed to dislodge them from Sana’a and the rest of the north.