Suspected al Qaeda-linked militants killed at least 18 Yemeni soldiers and security guards on Saturday in a car bombing and grenade attack on the intelligence service headquarters in Aden, the Defence Ministry and witnesses said.
The ministry said more bodies were believed buried under the rubble of the building, part of which was levelled in the attack in the southern port city. At least seven others were wounded before the militants fled.
The United States has been pouring aid into Yemen to stem the threat of attacks from al Qaeda in the Arabian Peninsula (AQAP) and to try to prevent any spillover of violence into neighbouring Saudi Arabia, the world's top oil exporter.
Last year, a U.S.-backed offensive drove al Qaeda offshoot Ansar al-Sharia (Partisans of Islamic Law) from cities they had seized in an uprising against the former president, Ali Abdullah Saleh.
But Islamist militants have carried out deadly suicide bombings on high-profile military and security targets since June, exposing the government's vulnerability.
In Saturday's attack, witnesses said militants had fired rocket-propelled grenades at the intelligence service's three-storey building, shattering windows and setting it on fire.
"The operation seemed to have been well planned," a local security source said. He said he believed the attackers belonged to al Qaeda.
He said the militants had stopped their vehicle in front of an adjacent five-storey television building, blown up a military vehicle guarding the compound, and then attacked the intelligence service building with automatic fire and rocket propelled grenades before fleeing.
The Defence Ministry said the militants had also detonated a car bomb next to the building, destroying part of it. Security sources put the death toll at 18.
The ministry earlier said all the casualties were members of the intelligence service and the Central Security forces, which guard the nearby television offices.
Ashraf Ali Ahmed, who lives in the area, said a loud explosion had shaken the area, followed by smaller blasts. "The blasts woke up the neighbourhood from their sleep," he said.
In July, militants attacked a police academy in Sanaa, assassinating the commander of the southern region, and trying to kill the commander of a tribal force allied with the army.
Washington has responded to the series of attacks by stepping up its drone strikes on AQAP, which was behind several failed attacks on the United States, including an attempt to blow up an airliner over Detroit on Christmas Day in 2009.