Terrorists must be denied the means to carry out a devastating nuclear attack, the UN secretary general has told an anti-terror summit in Madrid.
Kofi Annan and King Juan Carlos both addressed the conference
Kofi Annan was setting out a global strategy to fight terrorism which calls for preventive and deterrent measures - without sacrificing human rights.
Mr Annan said the time had come to outlaw terrorism in all its forms.
The summit comes nearly a year after the 11 March bomb attacks on Madrid trains which left 191 dead.
About 400 international experts and academics have spent three days discussing ways to combat terrorism while maintaining democracy.
Mr Annan said priorities included making it difficult for terrorists to travel, receive financial support and obtain nuclear material.
He urged UN member states to adopt the international convention on nuclear terrorism.
"Nuclear terrorism is still often treated as science fiction - I wish it were.
"But unfortunately we live in a world of excess hazardous materials and abundant technological know-how, in which some terrorists clearly state their intention to inflict catastrophic casualties," he said.
"Were such an attack to occur, it would not only cause widespread death and destruction, but would stagger the world economy and thrust tens of millions of people into dire poverty," he said.
"That such an attack has not yet happened is no excuse for complacency. Rather, it gives us a last chance to take effective preventive action."
He set out the five-point strategy as the need to:
- dissuade disaffected groups from choosing terrorism as a tactic to achieve their goals,
- deny terrorists the means to carry out their attacks,
- deter states from supporting terrorists,
- develop state capacity to prevent terrorism,
- defend human rights in the struggle against terrorism.
"It should be clearly stated, by all possible moral and political authorities, that terrorism is unacceptable under any circumstances and in any culture," Mr Annan added.
Spanish King Juan Carlos also addressed the summit on Thursday.
Among the tasks for delegates at earlier sessions of the International Summit on Democracy, Terrorism and Security, was to decide on a universally acceptable definition of terrorism.
The conference was organised by the Club of Madrid - a group of more than 40 former heads of state.
The experts are due to issue a Madrid Agenda, with guidelines on how to tackle the terror threat while preserving democratic values and traditions.
Organisers planned the conference to coincide with the anniversary of the Madrid attacks to "honour the courageous people of Madrid who have suffered immeasurable grief since the 11 March attacks and set out a way forward".
Twenty-two people have been jailed so far in connection with the bombings.
The attack was claimed by a Moroccan cell with links to al-Qaeda, and most of the arrested are Moroccan citizens.
On Friday, the final day of the event, a minute's silence will be observed to mark the anniversary of the attack, which injured 1,900 people.