The International Atomic Energy Agency (IAEA) released a report Tuesday saying it had credible intelligence Iran is seeking nuclear weapons technology.
"The agency has serious concerns regarding possible military dimensions to Iran's nuclear program," the IAEA report said.
"After assessing carefully and critically the extensive information available to it, the agency finds the information to be, overall, credible.
"This information indicates that Iran has carried out activities relevant to the development of a nuclear explosive device."
It added: "The information also indicates that prior to the end of 2003, these activities took place under a structured program, and that some activities may still be ongoing.
"Given the concerns identified above, Iran is requested to engage substantively with the agency without delay for the purpose of providing clarifications."
The nuclear watchdog said among the more than 1,000 pages of information it possessed indicated Iran has done work "on the development of an indigenous design of a nuclear weapon including the testing of components."
Previous IAEA reports focused on Iran's efforts to produce fissile material - which can be used for civilian and military purposes alike.
But the new update focuses on Iran's alleged efforts towards putting the radioactive material in a warhead and developing missiles.
Iran, which insists its nuclear program is peaceful, dismissed the new IAEA report prior to its publication as "laptop lies."
China and Russia had meanwhile pressured the IAEA not to even publish the report, diplomats said.
Russia, which has made billions of dollars helping Iran develop its nuclear program, has repeatedly used its veto at the UN Security Council to protect Tehran from universal sanctions.
Nonetheless, Western nations have put four rounds of partial sanctions on Iran's nuclear program – and speculation has swirled about a potential strike in Iran’s nuclear facilities.
Iran has become increasingly bellicose in response, and has threatened to strike Israel, Europe, and the US should its nuclear sites be attacked.