He also said the U.S. government is laying the groundwork to shift its massive oil spill clean-up operation from disaster management to long-term recovery.

Meanwhile, experts said drilling relief wells is a likely ultimate solution. The first of the two relief wells under construction is expected to be completed in August, yet it is still contingent on the weather.

The environmental disaster caused by the oil rig explosion that killed 11 workers in April has heavily affected the Gulf's fishing and tourism industries.

Some scientists warned the whole ecosystem in the Gulf of Mexico could be affected by the spill, while others said the effect could be minimal.

With the gusher having been sealed for two weeks, visible oil slicks are decreasing off the shores. But scientists cautioned that a large amount of spewed oil is still in the Gulf but its exact quantity and whereabouts remain unknown.

They feared that much of the oil had been trapped below the surface following the use of millions of liters of chemical dispersant, saying much now depends on nature's ability to eventually clean up the oil.

While racing against the time to contain the massive oil spill in the Gulf of Mexico, BP had to simultaneously deal with the legal mess left by the disaster.

At a hearing on Thursday in Boise, Idaho, lawyers for disaster victims argued over how piles of lawsuits against the British energy giant should be dealt with and where the trial should take place.