VIENNA — The head of the U.N. nuclear agency says that well over 100 cases of nuclear theft and related incidents are reported to his agency each year but many more may go undetected.

"We cannot be sure if such cases are just the tip of the iceberg," says Yukiya Amano, head of the International Atomic Energy Agency.

Amano spoke Monday at the start of an IAEA conference on nuclear security attended by government ministers and other senior officials from more than 100 countries.

He urged more IAEA member nations to ratify an agreement that obliges them to secure nuclear materials and facilities on their soil, saying 30 signatures are still needed to bring it into force.