Of course, it's not simply "Western powers" that believe this. As reported in The New York Times last February, it's also the official position of the UN International Atomic Energy Agency (IAEA):Iran says the [nuclear] program is only for peaceful purposes, but Western powers believe it is trying to produce nuclear weapons.
Reuters ran a similarly-worded story on the same day as the Times article but has since abandoned any mention of the IAEA findings in its coverage of the nuclear standoff. Instead, we get the banal boilerplate seen in Robert Evans' story linked above. This obfuscates both the evidence of, and near universal agreement that Iran is vigorously pursuing nuclear weapons.The United Nations’ nuclear inspectors declared for the first time on Thursday that they had extensive evidence of “past or current undisclosed activities” by Iran’s military to develop a nuclear warhead, an unusually strongly worded conclusion that seems certain to accelerate Iran’s confrontation with the United States and other Western countries.