The discovery of a hobbyist-style drone on the roof of Prime Minister Shinzo Abe's office on Wednesday has revealed a blind spot in Japan's counterterrorism policies and underlined the need for greater regulation of the increasingly popular aerial devices, experts said Thursday.

The government's failure to intercept the quadcopter drone, reportedly 50 cm wide and with a payload that tested positive for radiation, was "very problematic" from the viewpoint of counterterrorism policies, said Koichi Oizumi, a professor of crisis management studies at Aomori Chuo Gakuin University.

"The prime minister's office is supposed to be guarded with the most impregnable protection imaginable. Yet the government let the drone fall right smack on its roof," Oizumi said.