BP says its top kill operation is going to plan as it tries to plug the leak that the company estimates is leaking about 5,000 barrels of oil a day into the Gulf of Mexico.

Subsea efforts continue to focus on progressing options to stop the flow of oil from the well through interventions via the blowout preventer (BOP), and to collect the flow of oil from the leak points, the company said in a statement.

BP’s top kill method involves pumping heavy fluids or drilling mud into the well with the help of undersea robots, followed by putting cement on top of the well. It will take two days to evaluate its success.

Six hours into the operation, BP chief operating officer Doug Suttles announced that mud and not oil is gushing from the well.