The oil giant BP has said that the latest attempt to stop the Gulf of Mexico oil leak has failed. BP chief operating officer Doug Suttles said the firm was now shifting to a new strategy to stop the spill. In the failed procedure known as "top kill", the firm had been blasting waste material into a ruptured well after pumping heavy mud into it. The worst oil spill in U.S history began when a drilling rig exploded last month killing 11 people. The thick crude has already permeated more than 110 kilometers of Louisiana's coastline threatening fragile wetlands and putting the vital fishing industry at risk. BP says the next option – called the lower-marine-riser-package cap – involved an underwater robot using a saw to hack off the leaking pipe and place a cap over it. The operation is expected to last four days.