SANAA, Aug 5 (Reuters) - A bomb exploded near a police building in south Yemen, authorities said, and security forces defused a bigger device set to go off outside a government office in the region, where secessionist feeling runs high.

A blast blew a hole in a fence outside a criminal investigation department building in Abyan province but caused no injuries, the website of the ruling party quoted security sources as saying.

A larger device, made of a mine attached to a timer, was defused after being planted and set to explode near the fence of the offices of a government watchdog in Abyan.

"Official sources said ... outlaws were behind the two terrorist acts aimed at shaking security and stability in Abyan province," it said. "The suspects will be tracked down and arrested to be punished through the courts."

Gunmen killed four soldiers and wounded another last week in an ambush in Abyan, east of Aden -- the capital of the former People's Democratic Republic of Yemen which merged with its northern neighbour in 1990.

In July, at least another eight people were killed in clashes between security forces and armed men at an opposition gathering in Abyan to demand the release of detainees held during recent disturbances.

People in the south, home to most of Yemen's oil facilities, have long complained that northerners abuse the unity agreement to grab their resources and discriminate against them.

Yemen, one of the poorest countries outside Africa, has been battling a wave of al Qaeda attacks over the past two years and a Shi'ite rebellion in its north, as well as an increasingly restive south.

Neighbouring Saudi Arabia, the world's largest oil exporter, has said it fears instability in Yemen could allow it to become a launchpad for a revival of a 2003-2006 campaign by al Qaeda militants to destabilise the U.S.-allied ruling Al Saud family.

President Al Abdullah Saleh took power in the former North Yemen in 1978 and has been president of the new republic since 1990. He won another seven-year term in 2006. (Writing by Inal Ersan; editing by Andrew Roche)

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