ADEN: Suspected Islamist militants killed at least 14 Yemeni soldiers and security guards, and injured seven others, in a grenade attack on the intelligence service headquarters in the southern port city of Aden on Saturday, the Defence Ministry said.

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 seized in an uprising against former president Ali Abdullah Saleh.

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 opened fire on the intelligence building before fleeing.

A Yemeni security official said four officers guarding the intelligence headquarters had been killed and 10 others were wounded in the attack.

The Defence Ministry said later thatn the toll had risen to 14 dead and seven wounded. The casualties were members of the intelligence service and the Central Security forces, which guard the nearby television offices.

Suspected militants have carried out a series of deadly suicide bombings on high-profile military and security targets since June, attacking 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 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.