The power-unit monitoring system was not working properly when the control tower at Tokyo's Haneda airport suffered an electrical failure Tuesday because circuit breakers on the switchboard were being replaced, the transport ministry said.

Airport officials monitoring the power surveillance system were unaware of the malfunction to the uninterruptible power supply units until the outage hit because they had been told beforehand that the system would not function properly while circuit breakers were being replaced.

"We apologize for causing inconvenience to passengers," Teiji Iwasaki, chief of the Land, Infrastructure and Transport Ministry's Civil Aviation Bureau, said at a news conference Wednesday, adding that the ministry did not have adequate measures to detect problems in the UPS units during the breaker-replacement work.