Kenyan security forces appear to have launched an assault on the Westgate shopping complex in the capital Nairobi in an attempt to break the siege there.Gunfire and explosions were heard at the complex and a cloud of smoke could be seen billowing from the building.Television pictures showed troops running towards the complex. About 10 attackers were thought to be inside.More than 69 people have died and more than 170 have been injured since the attack began on Saturday.The Somali al-Shabab movement has said it carried out the attack.It said it was in retaliation for Kenyan military operations in Somalia.The latest round of gunfire and explosions appears to be a dramatic turn of events in the security forces' operation, the BBC's Mike Wooldridge reports from Nairobi.It is not clear as yet what caused the blasts, or what impact they are having on the operation to rescue hostages taken by the militants, our correspondent reports.More troops can be seen on the move around the building, he adds.