UPDATE 7-Islamist gunmen holed up with hostages in Kenya siege, 59 dead
* Somali Islamists linked to al Qaeda claim attack
* Group wants withdrawal of Kenyan troops from Somalia
* President loses relatives in raid, vows punishment
* Volleys of gunfire, helicopters swoop low over mall
By Richard Lough and Edmund Blair
NAIROBI, Sept 22 (Reuters) - Islamist militants were holed up with hostages on Sunday at a shopping mall in Nairobi, where at least 59 people have been killed in an attack by the al Shabaab group that opposes Kenya's participation in a peacekeeping mission in neighbouring Somalia.
Brief volleys of gunfire interrupted hours of stalemate in the late afternoon. A Reuters correspondent saw security personnel on the move and as dusk closed in two helicopters swooped low over the Westgate shopping centre, which has several Israeli-owned outlets and is frequented by many foreigners.
Mobile phone signals began to fail in the area as darkness fell. Kenya's president, vowing to stand firm against Somali militants, was cautious about the outcome, saying chances of the siege ending well were "as good ... as we can hope for". Continued...