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.

The misstep, he added, was a sobering reminder that Japan needs to beef up its counter-terrorism efforts ¡ª coming only a few months after the Islamic State group executed two Japanese hostages in Iraq or Syria. In one of its videos addressed to Abe, the militant group declared that Japan was now among its targets.

¡°Think what an awful disaster it might have been had the drone been equipped with a bomb or other explosives,¡± Oizumi said. ¡°Drones are now a terrible threat.¡±

He said Japan needs to implement stiffer legal measures around the use of drones, which have grown increasingly popular among the public as a platform from which to film aerial video.

Under the current aviation law, drone operators are in most cases not required to inform aviation authorities unless they plan to fly a device to an altitude of 250 meters or more.

Oizumi urged the government to pass stricter legislation against drones and other unmanned aircraft ¡°as soon as possible.¡±

The drone incident was ¡°disappointing,¡± said Kenzo Nonami, a professor of robotic engineering at Chiba University, adding that the purchase and ownership of drones be more rigidly controlled and supervised.

Some drones on sale, he said, are capable of flying several kilometers and higher than 200 meters. Individuals who purchase such high-spec drones, he said, should be registered in a database so that law enforcement can track down the owners in the event that a crime is committed.

Such rigorous preventive measures will not only help facilitate investigations but also serve as a deterrence against possible wrongdoing, Nonami added.

On Thursday at noon, visitors to a video-gaming section at a major electronics retail store in Tokyo were welcomed with an array of drones hanging from the ceiling, with price tags ranging from ?12,000 to ?450,000.

A flyer for one of them touted the aircraft as ¡°easily controllable¡± and capable of highly ¡°stable,¡± ¡°vivid¡± aerial shooting. Nowadays, some drone aficionados post their films on Youtube to make money, Nonami said.