Sixty-eight people have been killed and more than 170 injured since the attack began on Saturday.\nBetween 10 and 15 attackers - thought to be militants from the Somali al-Shabab movement - are still inside the Westgate shopping centre.\nReporters at the scene said there had been heavy and rapid bursts of fire.\nThe BBC's Mike Wooldridge in Nairobi says that developments reflect an intensification of efforts by the security forces to bring the stand-off to an end as conditions for those trapped in the complex deteriorate.\nAn unnamed Kenyan security source told the AFP news agency that an army assault was underway.\nAn AFP correspondent at the scene said he heard about 15 minutes of fierce gunfire which then subsided.\nA photographer accompanying the correspondent said troops deployed around the mall were forced to duck for cover.\nThe photographer said it \sounded as if the shots were coming from somewhere around the mall