Yemeni security forces clashed with dozens of pro-secession protesters in the southern port city of Aden, leaving at least three people injured on Thursday, a police officer told Xinhua.

The clashes erupted following the arrest of five youth activists of the pro-secession Southern Movement by security authorities earlier in the day in Aden, the local police officer said on condition of anonymity.

Troops of the security apparatus fired live ammunition and attempted to end the stone-throwing clash, the police officer said.

Crowds of secessionist protesters chanted, "No unity... No dialogue with Sanaa regime." They tried but failed to break through the security cordon surrounding the central jail in Aden to free their detainees, witnesses told Xinhua anonymously.

Extra police and soldiers as well as security force vehicles were deployed on Thursday evening nearby several government compounds in Aden, according to local residents.

The port city of Aden is a main stronghold of youth activists demanding outright independence for the south.

North and South Yemen unified peacefully in 1990, but the relationship deteriorated in 1994. Calls for separation in the country's southern regions were renewed in 2007.

The United States has become increasingly concerned that the conflicts in south Yemen are creating an unstable state where Al Qaeda could more freely operate to gain territorial foothold.