Suicide Bomber Attacks Yemen's Defense Ministry: Source

The defense ministry said the attack targeted the ministry's hospital and most of the gunmen had been killed or wounded.

"The attackers have exploited some construction work there to carry out this criminal act ... the situation is under control," the ministry said in a statement on its website.

No one claimed responsibility for the attack. But the US-allied country has been grappling with a security threat by al-Qaeda-linked militants, who have repeatedly attacked government officials and installations over the past two years.

Witnesses said the explosion shook the compound in the old district of Sanaa, where the country's central bank is also located.

"The attack took place shortly after working hours started at the ministry, when a suicide bomber drove a car into the gate," the defense ministry source said.

"The explosion was very violent, the whole place shook because of it and plumes of smoke rose from the building," an employee who works in a nearby building told Reuters.

Ambulance sirens and gunshots were heard after the blast as soldiers exchanged fire with the gunmen, said to have been disguised in Yemeni army uniforms, who had stormed the compound.

A military source said that at least 20 people, including militants, were killed in the attack and dozens were wounded. The Yemeni health ministry appealed to citizens to donate blood to help save the wounded.

At least two sources inside the defense ministry said the attackers came in two vehicles. One was driven by a suicide bomber who attacked the gate of the compound, while armed men entered the compound in the second, the sources said. The ministry statement made no reference to a suicide attacker.

Violence is common in Yemen, where an interim government is grappling with southern secessionists, al Qaeda-linked militants and northern Houthi rebels, as well as severe economic problems inherited from veteran President Ali Abdallah Saleh who was forced out of office in 2011.