The United States warned on Tuesday it would ratchet up pressure and ready new sanctions on Iran after the UN nuclear watchdog said its arch foe had worked on sensitive nuclear weapons technology.

The report prompted Republican hawks to demand 'crippling' economic retaliation from the White House, and came as rumors of a possible Israeli military strike on Tehran also assailed the White House.

A senior US official said Iran must answer concerns raised by the International Atomic Energy Agency (IAEA) and said Washington would consult with partners on "additional" pressure and sanctions on the Iranian government.

"We don't take anything off the table when we look at sanctions. We believe there is a broad spectrum of action we could take," the official said on condition of anonymity.

"We fully anticipate ratcheting up our pressure. We also want to make sure that what we are doing is to coordinate with other countries."

Another senior official said that the report echoed 'very serious concerns' in Washington about Iran's nuclear program.

He noted findings that Iran had carried out a 'structured' program under its ministry of defense from the late 1990s to 2003 on developing a nuclear warhead.

The official also expressed concerned that while the report did not say Iran had resumed that program, there were some indications that "activities of concern" could be continuing.

But the official also noted the report left questions about the current state of the program unanswered.

"The report does not assert, doesn't make any judgment about how advanced Iran is in their program," the official said.

"(It) certainly doesn't assert that Iran has mastered all the necessary technologies and we agree with that assessment."

The United States and its allies have long suspected Iran of pursuing nuclear weapons, which it is feared could threaten Israel and spark an atomic arms race in the Persian Gulf. But Iran has always denied the charges.

Washington already maintains a punishing range of economic sanctions against Iran, and officials have increasingly been arguing that the Iranian economy is now paying a real and painful price for the government's nuclear program.

But Obama's political foes want him to go further, and some are demanding US action against Iran's central bank which analysts said could effectively isolate the Islamic Republic from the global economy.

House Foreign Affairs Committee Chairwoman Ileana Ros-Lehtinen said Congress should "ratchet up the pressure" with new sanctions legislation to 'cripple' Iran's ability to pursue its nuclear program.

Republican Senator Mark Kirk called for Washington to take steps towards "collapsing the central bank of Iran" in light of the report.

"If this is not a smoking gun, I don't know what is," he said. Florida Republican Senator Marco Rubio also called for the White House to adopt a stiffer approach.

"The Obama administration should work with our allies and partners in Europe and Asia to impose crippling sanctions on Iran, including its central bank, to finally send a message that the Iranian nuclear weapons program is intolerable."

The administration's path to more onerous sanctions on Iran could be complicated however.

Russia and China expressed reservations about the publication of the IAEA report and may block a fifth round of United Nations sanctions in the Security Council.

The IAEA said in the report that it had "serious concerns" regarding possible military dimensions to Iran's nuclear program.

"This information indicates that Iran has carried out activities relevant to the development of a nuclear explosive device," it said.

The Vienna-based agency said some of its more than 1,000 pages of information indicated Iran has done work 'on the development of an indigenous design of a nuclear weapon including the testing of components'.

Previous IAEA assessments have centered on Iran's efforts to produce fissile material -- uranium and plutonium -- which can be put to peaceful uses like power generation, or be used to make a nuclear bomb.

But the update focuses on Iran's alleged efforts towards putting the radioactive material in a warhead and developing missiles.