United States president Barack Obama is warning that the economic impact from the Gulf of Mexico oil spill will be "substantial".

Mr Obama is trying to reassure Gulf coast residents that they will get through the oil spill crisis, but he warns there are tough times ahead.

"What is clear is that the economic impact of this disaster is going to be substantial and it's going to be ongoing," he said.

The official in charge of the US government's response to the oil spill, Thad Allen, says a cap on the ruptured oil well is collecting 11,000 barrels of oil a day, and that could soon increase to 20,000.

But he says the effects of the spill will be felt for a long time.

"Dealing with the oil spill on the surface ... will [take] years," he said.

The oil slick has now reached the shores of four states.

BP's clean up costs have now topped more than $1.5 billion.