A huge oil slick caused by an underwater leak continued to creep toward the U.S. Gulf Coast on Monday as the Obama administration pressed energy giant BP Plc to stem the oil gushing from its ruptured offshore well.

The direction of the slick has been pushed around by strong winds in the Gulf of Mexico while the likely economic and environmental costs of the accident mounted.

President Barack Obama visited affected communities on Sunday, pledging a “relentless relief effort” but keeping the focus on the British oil giant BP.

“Let me be clear: BP is responsible for this leak. BP will be paying the bill,” Obama said. “We are dealing with a massive and potentially unprecedented environmental disaster.”