SANAA: Two masked men on a motorcycle shot dead an Iraqi military adviser to Yemen's army on Tuesday, security and medical sources said, extending a series of killings bearing the hallmarks of al Qaeda.

The United States is worried that al Qaeda, entrenched in parts of Yemen will use a power vacuum to launch attacks abroad, and has stepped up drone strikes on suspected militants with the backing of President Abd-Rabbu Mansour Hadi.

Iraqi Brigadier General Khaled al-Hashemi, who works as a consultant at the Ministry of Defence, was gunned down near the foreign intelligence service building in the Yemeni capital Sanaa, a security source said.

"This operation is almost identical to the assassination of the security officer at the U.S. Embassy and has the fingerprints of al Qaeda (on it)," said the source, who asked not to be named.

Last week, Qassem Aqlan, a Yemeni man who worked in the security office of the U.S. Embassy in Sanaa, was killed by gunmen in a drive-by shooting.

Hashemi was part of a team tasked with restructuring the Yemeni army after the popular uprising that ousted longtime strongman president Ali Abdullah Saleh in February.

He was one of several Iraqi military experts hired by the Sanaa government after the toppling of Saddam Hussein in 2003, a government source said.

Tuesday's attack was the latest in a series targeting security officials and politicians in the impoverished and often chaotic Arabian Peninsula state, which is battling Islamist militants with U.S. assistance.

Yemen-based Al Qaeda in the Arabian Peninsula (AQAP) and other militant groups seized parts of the country during the anti-Saleh revolt.

The Yemeni-U.S. offensive drove Islamist fighters out of several southern towns earlier this year, but militants have struck back with a series of bombings and killings.

On Monday night, unknown attackers hurled a hand grenade at the home of Magli Ahmed, the military police chief in Sanaa, according to the Defence Ministry. Ahmed was unharmed but a taxi driver was killed in a shootout that followed the blast, a security source said.

Restoring stability in Yemen has become an international priority due to fears that al Qaeda could become further entrenched in a country which flanks oil producer Saudi Arabia and lies along major international shipping lanes.

Security has continued to suffer from a power struggle in security and military units as Hadi tries to remove figures close to Saleh who still head many state bodies.