The world has seen a "big achievement" in barring nuclear terrorism over the past six years but the threat remains, warned director general of International Atomic Energy Agency (IAEA) Yukiya Amano.

Speaking in Washington DC, Amano said that preventing extremist groups from obtaining nuclear material is one of the major concerns being discussed at the Nuclear Security Summit 2016. The director general said that the risk should not be ruled out even though no nuclear attack by terrorist groups has ever been successfully launched.

"Until now, there has not been a terrorist attack using nuclear material," Amano said. "But this fact should not blind us on the possibility of terrorists using nuclear material in the form of a dirty bomb, for example. We should stay very vigilant and the possibility is real, the risk is there. So we need to cooperate among member states; each country needs [to] enhance their nuclear security."

Amano attributed this achievement in nuclear security to the enhanced efforts from the international community.

"Since the 9/11 attacks the efforts by member states have increased a lot," said Amano. "Even in [the past] six years or seven years, there has been big achievement. For example, the IAEA has trained over 10,000 people in the past 10 years.

"And we have donated some 3,000 [pieces of] equipment to all the countries, and we have collected some 3,000 [pieces of] information in database. So these efforts have been strengthened and we should not forget about the strengthened efforts of member states to fight against the nuclear terrorism."

Commenting on the United Nations' 2,270 resolutions on North Korea's nuclear test, Amano said that the country's action "was a clear violation of the Security Council resolutions, and members of the Security Council reacted in accordance with their mandate."