Incidents of late-night noise pollution caused by the U.S. Kadena Air Base in Okinawa Prefecture rose to a monthly average of more than 160 cases between April and July, Kadena town officials said Friday.

The frequency with which noise levels have exceeded 70 decibels between late at night and dawn has continued to rise over the past few years, they said. In fiscal 2000, the monthly average was 80 times, rising to about 150 in fiscal 2001.

The town believes aircraft taking off and landing is the main cause of the noise. The base is home to the largest U.S. Air Force unit stationed overseas.

"Late-night noise increased after the Sept. 11 hijacking attacks on the United States last year, especially around February this year," a town official said.

Japan and the U.S. agreed in March 1996 that flights to and from the U.S. base between 10 p.m. and 6 a.m. would be limited to those "essential" to operations.

Recently, there has been a series of emergency landings at the Kadena base, including one by a P-3C antisubmarine patrol aircraft on Thursday and by several F-15 jet fighters earlier, according to Kadena town officials.