UPDATED:

SANAA: A suicide attacker set off a massive car bomb in Yemen's southern city of Aden on Monday, killing at least 45 pro-government troops who had been preparing to travel to Saudi Arabia to fight Houthi rebels in Yemen's north, officials said.

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. They spoke on condition of anonymity because they weren't authorized to brief journalists.

Yemen is embroiled in a civil war pitting the internationally recognized government and a Saudi-led coalition against the Shiite rebels, who are allied with army units loyal to a former president. The fighting has allowed al-Qaida and an Islamic State affiliate in Yemen to expand their reach, particularly in the country's south.

No group immediately claimed responsibility for the bombing.

The U.N. 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.

Death toll in Yemen’s Aden suicide bombing rises to at least 45: MSF

ADEN: The death toll in a suicide bombing at a local militia compound in the southern Yemeni city of Aden rose to at least 45, medical charity Medicins Sans Frontieres said on Monday.

Yemeni security officials say a suicide attacker set off a massive car bomb in the southern city of Aden, killing 45 pro-government troops who had been preparing to travel to Saudi Arabia to fight in Yemen's northern border area.

They say the Monday attack left over 60 wounded, who have been taken to three area hospitals. The men had been at a staging area near two schools and a mosque where they were registering to join the expedition, which the Saudis seek to build up to 5,000 men who will also receive training.

Yemen is embroiled in a civil war pitting the internationally recognized government and a Saudi-led coalition against the rebels and army units loyal to a former president.