* U.S. monitoring finds 'diminished concentration' of oil
* Response workers down to 25,000 from peak near 48,000
HOUSTON, Sept 21 (Reuters) - The U.S. government is unable to confirm reports of a miles-long plume of oil lurking beneath the surface of the Gulf of Mexico from BP Plc's (BP.L) (BP.N) giant oil spill, a government scientist said on Tuesday.
The government confirmed on Sunday that BP had permanently "killed" its deep-sea well that ruptured in April and unleashed the worst spill in U.S. history.
Some private scientists and academic groups say sizable amounts of oil remain trapped deep beneath the ocean surface, after a government report in August found that more than half of the 4 million barrels spewed by the well had dissipated, and much of the rest had been neutralized by natural processes.
Undersea monitoring by the National Oceanic and Atmospheric Administration failed to detect significant concentrations of oil, NOAA scientist Sam Walker said.
"We are continuing to find lower and lower concentrations," Walker told reporters on a conference call.
In recent weeks, U.S. monitoring vessels have seen "very clear trends of diminished concentration," with oil detected beneath the surface in the "parts per billion" level, "which is not actionable," Walker said.
Walker said U.S. officials "respect the independence" of private researchers at the Woods Hole Oceanographic Institution in Massachusetts, who published a report in the Aug. 19 edition of the Journal science claiming to have found a 22-mile-long (35-km-long) underwater oil plume near the BP well.
Walker said the Woods Hole data was collected in June, and that conditions had likely changed dramatically since then. "We are not finding the concentrations that they found," he said.
The U.S. government has scaled back its Gulf coast response efforts after a provisional cap sealed the BP well on July 15. The effort is utilizing 25,000 workers, down from about 48,000 at the height of the spill response.
The government has closed posts in Houma, Louisiana, and Mobile, Alabama, to consolidate staff at a New Orleans command center, which will oversee long-term recovery efforts.
The command center will be run by Coast Guard Rear Admiral Paul Zukunft, who will replace retired Coast Guard Admiral Thad Allen as the government's point-man for the spill response.
About 600 miles (970 km) of coastline is still affected by the spill -- including an elaborate system of coastal marshes and bayous, Zukunft said.
The government on Tuesday opened nearly 8,000 square miles (21,000 sq km) of the Gulf of Mexico to fishing, which is about 20 percent of the federal waters that had remained closed to fishing due to oiling concerns. (Editing by Mohammad Zargham)