A U.N. nuclear watchdog team says Japan may need longer than the planned 40 years to decommission its tsunami-crippled nuclear plant and is urging its operator to improve plant stability.

The head of the International Atomic Energy Agency team, Juan Carlos Lentijo, said Monday that damage at the Fukushima Dai-ichi plant is so complex that it is "impossible" to predict how long the cleanup may last.

He suggested it would take longer than the 40 years Japan has projected.

The plant runs on makeshift equipment and frequently suffers glitches.

Lentijo urged the plant operator to promptly replace temporary equipment with a more reliable, permanent system.

The 12-member mission plans to release a report next month.

The plant was badly damaged by an earthquake and tsunami in March 2011.