The top official overseeing the U.S. response to the Gulf oil spill says he does not know how long it will take for the ruptured well to be plugged for good.

Retired Coast Guard Admiral Thad Allen told reporters Wednesday that efforts to dig a relief well have been put on hold due to what he called "an overabundance of caution."

BP has already sealed the top section of the broken well with a combination of mud and cement.  The U.S. government and BP are concerned that using the same method to seal the bottom of the well could cause a build-up in pressure and possibly force more oil to leak into the Gulf of Mexico.

Allen says the relief well will only be completed once the government and BP are confident all possible risks have been addressed.

Also Wednesday, the U.S, government defended its estimates that most of the oil that leaked into the Gulf had been captured, burned or dispersed and that only about a quarter of the oil is still in the water.

That figure had been questioned by scientists at the University of Georgia.  They said a review of government data showed almost 80 percent of the oil from the BP leak remains in the Gulf.

The BP disaster is the worst offshore oil spill in history and has severely impacted the fishing and tourism industries of the Gulf Coast.

Oil first began gushing into the Gulf of Mexico after an April 20 explosion on a rig leased by BP, which ruptured the oil well and killed 11 workers.  

Meanwhile, BP is preparing to transfer responsibility for damage claims from the Gulf oil spill to an independent facility.

Wednesday is the last day BP will accept claims from individuals and businesses. After that, the British-based oil company will direct those affected by the spill to the Gulf Coast Claims Facility, which starts work Monday.

BP has established a $20 billion fund for compensation payments and says it has already paid $368 million to people and businesses that suffered in the disaster.

The company says it will continue to handle claims from government entities and take calls about suspected fraud.

Some information for this report was provided by AP, AFP and Reuters.