The Institute for Science and International Security, a private group opposed to nuclear proliferation, called for tougher US economic sanctions against Iran and pressure on major trading partners to isolate Tehran.

The group looked at Iran's "critical capability," defined as the point at which the clerical regime will be able to produce enough weapons-grade uranium or separated plutonium to build one or more bombs before foreign detection.

"Based on the current trajectory of Iran's nuclear program, we estimate that Iran could reach this critical capability in mid-2014," the think tank said in a report.

The think tank based its assessment on the growth in Iran's stockpile of enriched uranium and number of centrifuges and what it described as an uncooperative stance by Tehran toward the UN atomic agency.

The institute said it was "deeply skeptical" of the potential for preventing Iran from developing nuclear weapons and painted a dire picture of the consequences if the regime developed the bomb.

The think tank said that a nuclear weapon would "embolden Iranian aggression and subversion" and questioned whether Iran's leadership, with its "apocalyptic messianism and exaltation of martyrdom," could be deterred from using a bomb.

The report also said that an Iranian nuclear arsenal could motivate Saudi Arabia to develop a nuclear program, fueling proliferation in a region where Israel is the sole, albeit undeclared, state with nuclear weapons.

The United States has championed sanctions aimed at crippling the Iranian economy by cutting off its oil exports, while Israel has not ruled out the possibility of a military strike on Iran.

Critics of the approach say that sanctions are hurting ordinary Iranians and that the Iranian leadership, drawing lessons from Pakistan and North Korea, wants to use the threat of going nuclear to ensure the regime's own survival.

US intelligence agencies have not concluded that Iran has decided to build a nuclear bomb, saying only that Tehran is keeping the option open. Iran says that its nuclear program is for civilian purposes.