US President Barack Obama has pledged to treble the number of emergency crews working to clean up the Louisiana coastline. He made the announcement during a visit to the disaster area. BP says it may not know until Sunday whether its "Top Kill " operation to stop the leak has been successful. According to government figures between 70 and 112 million litres of oil have spilled in to the ocean so far - that's almost three times as much as the 1989 Exxon Valdez disaster.