Yemeni officials say 52 people have been killed in a suicide car bombing and a subsequent attack by gunmen on the Defense Ministry in the capital, Sana'a.
A statement by the Supreme Security Commission said 167 people were also wounded in Thursday's attack, the deadliest in more than a year.
Casualties include soldiers, staff at a military hospital inside the Defense Ministry's heavily guarded complex in central Sana'a, and civilians.
Officials said the bomber rammed a car packed with explosives into one part of the complex as attackers on foot opened fire with automatic rifles on another.
Several foreign medical workers - some of them Germans - are reportedly among the dead after the attackers stormed the hospital.
One witness said "The heavy gunfire on the hospital was shocking."
A second witness said "We first heard the sound of gunfire then we realized that there were people in military costume throwing bombs inside the hospital."
Yemen's President Abd-Rabbu Mansour Hadi later visited the compound, met senior military officials and ordered an investigation into the attack.
Reuters reported that a relative of Mr. Hadi was among those killed.
There was no immediate claim of responsibility, but al-Qaida militants often attack Yemeni security forces. The Yemeni branch, known as Al-Qaida in the Arabian Peninsula, is considered one of the world's most active.
Yemeni officials say the group has infiltrated the country's security services, and there were reports the attackers wore military uniforms.