Two separatists were also shot and wounded in another southern province when security forces opened fire to disperse protesters demanding the release of separatist prisoners, witnesses said.

Tensions between Yemeni security forces and southern secessionists protesting against President Ali Abdullah Saleh's government have been on the rise in recent months, accompanied by deaths and widespread arrests on both sides.

The car bomb that killed the retired officer was the latest in a string of killings in Shabwa province, where a soldier was shot dead at a police post on April 4, the same day a separatist activist and his son were killed on their farm.

Yemeni officials told Reuters the retired officer was killed by a bomb placed under the driver's seat of his car on Wednesday night. While he was retired, his brother continued to work with security forces.