A gunman shot dead Sunday the head of a Yemeni liberal political party in Sanaa, his family said, as Al-Qaeda suspects killed 18 troops and captured 15 others further west.

Secretary general of the Union of Popular Forces party Mohammed Abdulmalik al-Mutawakil was shot dead by a gunman on a motorbike on a street in central Sanaa, his daughter Radhia told AFP.

Mutawakil was "fatally shot in the neck," by his assailants who fled, she said.

A medical source confirmed the death.

It was not immediately clear why Mutawakil was targeted.

The attack came a day after Shiite rebels and their political rivals, including Mutawakil's party, signed an agreement on the formation of a new technocrat government to resolve the country's crisis.

Meanwhile, further west in the Red Sea port city of Hudeida, suspected Al-Qaeda militants killed 18 Yemeni troops and captured 15 others in an attack, security and tribal sources said Sunday.

The overnight attack targeted security headquarters in Hudeida, which Shiite Huthi rebels seized in mid-October as they expanded across several towns and cities with almost no resistance from government forces.

Al-Qaeda frequently carries out attacks on Yemen's security forces and it has been locked in deadly battles with the Huthi rebels.

Dozens of Al-Qaeda suspects took part in the attack on the security forces headquarters in Jabal Ras, in Hudeida, "killings 13 soldiers, wounding 10, and capturing 15 others," a security official said.

The assailants seized the building but three of the militants were also killed, the official said.

Army reinforcements were sent to Jabal Ras and negotiations launched with the assailants to withdraw from the building, a tribal dignitary said.

"Tribal mediation is underway to secure the withdrawal of Al-Qaeda militants and the release of the captured soldiers," the source told AFP.

Yemen is a key US ally that has allowed Washington to conduct drone strikes against Al-Qaeda on its territory.

The Huthis easily overran Sanaa in September before moving on to Hudeida as well as Shiite-populated Dhamar and neighbouring Ibb.