The United Nations nuclear watchdog has ended its 12-year investigation into possible military dimensions of Iran's nuclear program, concluding that Tehran conducted rudimentary nuclear weapons-related research until 2009.

The International Atomic Energy Agency's 35-nation board of governors unanimously passed a resolution on Tuesday that will officially close the investigation in the coming weeks, removing the main obstacle to implementing the July 14 nuclear deal between Iran and six world powers. The accord stipulated that the probe must be formally ended before international sanctions on Tehran are lifted, reports CBS News.

IAEA chief Yukiya Amano said before the resolution's adoption that Iran had conducted "a range of activities relevant" to making nuclear weapons and admitted that his investigation couldn't "reconstruct all the details of activities conducted by Iran in the past."

The agency's report circulated this month made it clear that Iran had failed to clearly answer questions about its past nuclear efforts, but despite the findings, the international community has been relatively quiet, suggesting a wish to proceed with implementing the accord, according to Reuters.

Iran has repeatedly denied pursuing a nuclear weapon and insists that its program is entirely for civilian purposes.

Iranian Foreign Minister Javad Zarif welcomed Tuesday's announcement, saying it proved the peaceful nature of its program.

"This resolution goes far beyond closing the issue of so-called PMD (Possible Military Dimension) and cancels the 12 previous resolutions of the council of governors of the IAEA which seriously restricted our country's nuclear program," Zarif said, according to the BBC.

Iranian President Hassan Rouhani said Wednesday that "Iran will start implementation of the nuclear deal within two or three weeks" and voiced hope that sanctions on Iran would be removed in January so that he could fulfill "one of the electoral promises of the government."

Amano said Wednesday that it is possible for the deadline to be met if Iran is able to implement the required nuclear restrictions in a timely manner.

Before sanctions can be lifted, Iran must dismantle about 12,900 of its 19,000 uranium-enriching centrifuges and ship most of its enriched uranium to Russia in exchange for a less refined form of uranium known as yellowcake, to ensure that it is unable to covertly divert nuclear energy to bomb making. It must also remove the core of the Arak heavy-water reactor, rendering it incapable of producing weapons-grade plutonium.

Once the IAEA can confirm that all the restrictions on Iran's nuclear program are in effect, the crippling economic sanctions will be lifted.