SEOUL — U.S. Forces Korea officials confirmed Wednesday they had returned closed military facilities to South Korea, a week after South Korean activists rallied to protest the June 1 transfer of nine camps.

The closed facilities are the last of 24 camps, closed between 2004 and mid-2006, that South Korea last summer agreed to accept from the United States.

The U.S. had spent as much as $400,000 a month last year guarding the empty bases while waiting for the transfers to take place, USFK spokesman David Oten said.

The United States has pushed in recent years to return closed bases, valued at more than $1 billion, as part of the transformation of its forces on the peninsula.

South Korean officials initially refused to accept them because of environmental concerns.

According to the status of forces agreement between the two countries, USFK must return land it has been using to South Korea when it is no longer needed.

That agreement says USFK can return the land without removing pollutants beyond those posing “known, imminent, and substantial endangerments to human health.”

U.S. officials announced in April 2006 that they would go beyond those requirements and remove underground fuel tanks at all returned bases, heavy metals from firing ranges and fuel contamination of groundwater at five camps.

South Korea officials agreed to accept 15 closed facilities in July 2006, and issued a statement that said negotiations had fallen short of their expectations, but they tried to get the best outcome.

Twenty South Koreans gathered last week outside the Ministry of Foreign Affairs to demand a full-scale environmental cleanup of the returned bases.

USFK officials have said they will return 59 camps totaling more than 33,000 acres in the next three years.