NATO Secretary-General Jaap de Hoop Scheffer said on Monday he was not certain the world can stop Iran from building a nuclear bomb.
Scheffer told a conference in southeast France that NATO did not have a direct role to play in the issue, but said he was worried that the United Nations had failed to curb Iran's nuclear ambitions.
"It is a major challenge to prevent Iran from continuing to strive to get the bomb," Scheffer told a World Policy Conference organised by France's IFRI foreign affairs think tank.
"I am not positive about the world being able to stop Iran from fulfilling its ambitions," he added.
Iran says its uranium-enrichment programme is only for electricity generation, but is under United Nations' sanctions over past undeclared activity and its failure to prove its intentions are wholly peaceful.
The UN Security Council last month again ordered Iran to "comply fully and without delay" to demands it stop enrichment, but failed to introduce any new sanctions as sought by the United States and its Western European allies.
"My concern is that the Security Council, as we speak, is rather incapable of coming to further conclusions on further sanctions," Scheffer said.
As a result of Iran's nuclear activities, Scheffer said he would never expect Israel to abandon its own, "supposed nuclear arsenal." Israel is thought to have the Middle East's only atomic weapons, but has never formally said so.
"As we all know, Israel never admits to what it has, but I do not see very many arguments for the Jewish state to abandon its potential," he said.
Scheffer added that he was equally concerned about Iran's drive to develop its missile capabilities.
"What is as dangerous (as the nuclear programme) is the missile technology which (Iran) is also developing at a fast pace," he said. "This becomes an element for the security of the United States and of course Europe.
Iran has a series of medium-range missiles, which experts say could target Israel. Tehran has also said it is working on long-range missiles.