WASHINGTON, Aug 4 (Saba) -- Impact from the massive oil spill in the Gulf of Mexico could last "for years and possibly decades to come," even after BP successfully capped the leaking well, a top U.S. administration official said Wednesday, according to Kuwait News Agency (KUNA).

"We remain concerned about the long term impact," Jane Lubchenco, head of the National Oceanic and Atmospheric Administration, said at a White House briefing. Obama administration officials say real progress has been made after British Petroleum (BP) began pumping mud into the damaged pipeline to stop the flow of oil in what is known as a "static kill.

Retired admiral Thad Allen, who oversees the government's response to the Gulf oil spill, said at the same briefing, he has "high confidence" no more oil will leak from the BP site and the chances of killing the well outright have "significantly improved." After BP finished the final steps of pumping mud and cement through separate relief wells that have been drilled down to the site, Carol Browner, presidential adviser on energy and climate change, says "there remains a lot to be done" to restore the Gulf region.

The latest scientific research estimates 17 percent, or 827,000 barrels of oil were recovered directly from the well site, while an additional five percent were burned and three percent skimmed using fishing boats. Remaining 26 percent of remaining oil is rapidly degrading naturally, including 27,000 tons of tar balls that have washed ashore beaches along the Gulf of Mexico.

According to Lubchenco, at least 50 percent of oil that was released is now completely gone from the system.

Meanwhile, the White House says cleanup results for the oil spill in the Gulf of Mexico would have been different if federal officials had not pushed BP "at every step of the way" to do things faster and more comprehensively White House spokesman Robert Gibbs said that no one owes departing BP chief executive Tony Hayward an apology despite positive news that cleanup efforts in the Gulf of Mexico are working.

Hayward is stepping down in October and taking a post at a BP joint venture in Russia after facing criticism for his handling of the spill and cleanup efforts.

Saba