* Yemeni demonstrators condemn killings
* Yemen expands search for kidnappers
SANAA, June 17 (Reuters) - Thousands of Yemenis marched on Wednesday in protest at the killing of three foreign hostages as security forces expanded the search for six still held by kidnappers.
Groups of students, clerics, politicians, tribesmen and state employees carried banners across Saada, the centre of the mountainous province where the nine were seized last week in an attack that an analyst said bore the hallmarks of al Qaeda.
The killing of the three women coincided with a rise in separatist and militant tensions in a country whose instability has alarmed Western countries and Saudi Arabia.
"Islam rejects aggression and crime," chanted some marchers, while others carried a banner reading "Those behind the attacks have no conscience".
State media said security forces backed by helicopters had expanded the search for the kidnappers into three neighbouring provinces.
Yemen has pledged to hunt down those behind the killing of the hostages, identified by officials as two German nurses and a Korean teacher, and offered a reward of $275,000 for information leading to the capture of the kidnappers.
The nine comprised seven Germans, a Briton and a Korean, according to state media, and included three children and their mother.
Yemeni authorities have blamed the Houthi tribal group, who belong to a Shi’ite Muslim sect, for kidnapping the nine foreigners, a charge the Houthis have denied.
But Abdulkarim Sallam, a researcher at Yemen’s Saba Centre for Strategic Studies, said al Qaeda may be behind the kidnapping.
"It is too early for a final judgment, but the Houthis have not been known to carry out such actions ... and tribes usually kidnap foreigners to pressure the government," Sallam said.
"Therefore I think the fingerprints of al Qaeda are clear in this operation because it is similar to their earlier operations." If the killings were carried out by tribesmen, it would be the first time that female hostages have been their victims. However, two Belgian women were killed in 2008 by gunmen in an ambush authorities blamed on al Qaeda.
Yemen last week arrested a man described as al Qaeda’s top financer in Yemen and Saudi Arabia. Yemen, the Arab world’s poorest country, is struggling with a revolt in the north, a secessionist movement in the south and growing militancy.
The unrest has raised concerns Yemen could slip into chaos and provide a base for al Qaeda or pirates operating in the Indian Ocean. (Reporting by Mohamed Sudam; writing by Firouz Sedarat)