Militants who carried out a brazen day-light assault on the Yemeni defence ministry that killed 56 people were mostly Saudis, investigators said on Saturday.

The finding, in a preliminary report on the inquiry into Thursday's attack, adds credibility to a claim of responsibility by Al-Qaeda in the Arabian Peninsula (AQAP), which was formed from a merger of the jihadist network's Saudi and Yemeni branches.

The investigators said the assailants wore military fatigues and penetrated the sprawling Sanaa complex in the confusion created when a suicide bomber rammed a vehicle into the gate.

They said the bodies of 12 attackers were recovered after security forces regained control.

A hospital in the complex took the brunt of the assault. Among the dead were medics from the Philippines, Germany, Vietnam and India. Civilian patients as well as soldiers were also killed, Yemen's supreme security committee said.

Al-Qaeda claimed responsibility Friday, alleging that the complex hosted US personnel behind drone strikes against its militants. There was no immediate evidence to support the claim.

Yemeni analyst Saeed al-Jamhi, who specialises in Al-Qaeda affairs, said the attack reflects "the level of the network's penetration into security and military services" and shows it has access to "high-level information".