Louisiana/Washington, May 3 (DPA) US President Barack Obama Sunday warned of a "massive and potentially unprecedented environmental disaster", as the widening oil slick in the Gulf of Mexico edged to 14 km off the coast of Louisiana.

But Obama said his administration would continue to mount a "relentless response" to the oil spill, the result of the sinking of an offshore rig April 20.

"We will spare no resource to clean up whatever damage is caused," Obama said in Venice, a staging ground for efforts to fight the oil slick, after meeting state and federal officials in Louisiana.

"From day one we have prepared and planned for the worst, while hoping for the best," Obama said, adding that his administration "would not rest" until the leak is stopped at the source, the oil is cleaned up and people in the region are able to go back to their livelihoods.

However, he also made it clear that rig owner BP Plc would bear all the costs. "BP is responsible for the leak," Obama emphasised, "BP will be paying the bill".