ADEN, Yemen - Security forces and armed southern separatists clashed on Monday in Yemen's port city of Aden, witnesses said, as rising tensions in the country's south threatened to disrupt Tuesday's presidential vote. 

Troop reinforcements with dozens of armored vehicles arrived in Aden from the capital Sanaa late on Sunday to reassure voters after hardline members of the separatist Southern Movement threatened violence on election day, security officials said. 

Witnesses said government troops and armed separatists exchanged fire mid-morning on Monday in the Mansura neighborhood of Aden, a stronghold of the movement, where a mass protest against the poll was expected later in the day. 

There were no immediate reports of casualties. 

Authorities are taking pre-emptive action, with a security official saying that police on Sunday and Monday had carried out "arrest raids on armed hardliners" from the Southern Movement who were trying "by force to prevent citizens from participating in the elections". 

"These elements are trying to create a state of fear among citizens by spreading rumors that Feb 21 (election day) will see acts of violence," the official said, speaking on condition of anonymity. 

Al-Qaida revenge 

A Yemen-based al-Qaida wing vowed on Monday to seek revenge against the Yemeni transitional government after its local chief was assassinated last week by a government agent. 

"Sheikh Tariq al-Dhahab, chief of the al-Qaida group in Radda town in al-Bayda province, was killed by the Yemeni government," the militant group said on Monday in a statement posted on Jihad-run websites. 

"We urge all Yemeni tribal chieftains to follow the strong tribal leader al-Dhahab in supporting al-Sharia (Islamic Law) and join al-Qaida, in order to work together to achieve justice and security, as we almost completely control and implement the Islamic Law in southern provinces of Abyan and Shabwa," it said in the statement obtained by Xinhua. 

Al-Dhahab, a brother-in-law of the slain Yemeni-born US cleric Anwar al-Awlaki, was assassinated last Thursday in a mosque in Radda by his half-brother Hizam in an operation planned by the Yemeni intelligence apparatus, according to security officials in Sanaa.