ADEN (Reuters) - A suicide bomber killed himself and a bystander in south Yemen on Saturday in an attempt to assassinate a government official who had targeted al Qaeda militants, a security source said.

The attacker walked up to the parked car of Mohammed Aidarous, head of a "popular committee" of tribal volunteers who helped the army oust militants from the town of Lawdar in Abyan province this year.

Aidarous, the local government official in charge of Lawdar, was not in the vehicle, but a bystander was also killed in the blast, the source said. Four people in the car and five passers-by were injured.

The source identified the bomber as 20-year-old Ali Mohammed Hossain from Abyan province.

Yemen has been in turmoil since an uprising against Ali Abdullah Saleh, who finally stepped down as president in February. The Arabian Peninsula state borders top oil exporter Saudi Arabia and lies on major world shipping lanes.

The army has expelled militants from Abyan towns occupied last year. But al Qaeda has struck back with assassination attempts against officials, some successful, and a suicide bombing of a military parade in May that killed 100.

Four members of Saleh's General People's Congress (GPC) were killed in an ambush outside Sanaa on Friday night, the party said on its website. Eight were wounded.

It was not clear if the group was targeted because of its party affiliation. A tribal source in al-Jawf where some of the men worked said the ambush could have been a tribal vendetta.

Saleh remains head of the GPC, which retains half the seats in cabinet. His relatives control key military and security units.

Saleh was succeeded by his deputy Abd-Rabbu Mansour Hadi, who has U.S. backing for a transition process that tries to balance the interests of groups including a Shi'ite Islamist movement in north Yemen, southern secessionists and tribal and Sunni Islamist groups who benefitted under Saleh's rule.

(Reporting by Dhuyazen Mukhashaf and Mohammed Ghobari; Writing by Andrew Hammond; Editing by Angus McDowall in Riyadh and Jason Webb in London)