* New unrest in south Yemen, days after six shot dead
* Party says HQ torched in southern town (Adds deaths, arson attack on party headquarters)
ADEN, Feb 23 (Reuters) - Two people were killed and nine others were wounded in clashes between Yemeni security forces and separatists in the south of the country on Saturday, medical and security sources said.
The clashes followed separatist demonstrations in Mukalla, Ghayl ba Wazir and Aden, capital of the former state of south Yemen which merged with the north in 1990. Six people were shot dead during protests on Thursday.
One of those who died on Saturday was a 50-year-old civilian in Aden who was hit by a bullet, the sources said. The other was killed in Ghayl ba Wazir, a small town near Mukalla. The protests and clashes ended around noon.
The Islah party, one of the most powerful in Yemen, also said on Saturday that separatists had set fire to its headquarters in Mukalla, the capital of a former south Yemeni province.
The resurgent movement for a revived south Yemen state has aggravated political instability in the Arabian Peninsula country, where Washington fears political chaos is giving al Qaeda space to operate.
Yemen also faces an insurgency from Islamist militants linked to al Qaeda in some southern areas and a rebellion by the Shi'ite Muslim Houthi movement in the north.
North and south Yemen were unified in 1990 after the Communist-led southern government collapsed. Northern forces won a brief civil war four years later after the south tried to secede from the union.
The secessionist movement gained strength during mass, nationwide street protests against former president Ali Abdullah Saleh in 2011, which forced the veteran strongman from office a year ago.
On Thursday, security forces shot at dozens of secessionists in Aden as they staged a rally against Saleh's successor, President Abd-Rabbu Mansour Hadi, a southerner who heads Saleh's party. There were also armed clashes in another southern town, al-Dalea.
Medical sources and witnesses said at least six people had been killed during the two clashes.
Southern Yemenis complain of discrimination by the government in the north. Tackling lawlessness in the country, which lies near oil shipment routes and flanks the world's top oil exporter, Saudi Arabia, is a priority for both Western and Gulf countries. (Reporting by Mohammed Mukhashaf; Writing by Angus McDowall; Editing by Mark Trevelyan)