While Iran reacted scornfully to the unilateral US sanctions announced Thursday, Washington insisted that they were a wake-up call both to the clerical regime and the international community.

But analysts questioned the sanctions' effectiveness in the absence of concerted UN action against Iran, and opposition Democrats saw worrying parallels to the drumbeats of war that preceded the 2003 invasion of Iraq.

"Every day now, it seems that the confrontational rhetoric between the United States and Iran escalates," senior Democratic Senator Robert Byrd said.

"President (George W.) Bush needs to understand that the Congress will not be kept out of the loop while his administration plots another march to war."

The sanctions targeted the Islamic Revolutionary Guard Corps, accused of spreading weapons of mass destruction (WMD), and the IRGC's elite Quds Force, which was designated as a supporter of terrorism.

Three Iranian state-owned banks were also blacklisted, along with IRGC-controlled companies and the logistics arm of Iran's defense ministry, as the United States stepped up a drive to squeeze Iran out of global banking.

Through ever-stricter sanctions, Washington has tried and failed to exert pressure on Iran ever since the US embassy hostage crisis that erupted following the 1979 Islamic revolution.

Responding to Thursday's move, Iranian foreign ministry spokesman Mohammad Ali Hosseini said the United States would fail again.

"The hostile American policies towards the respectable people of Iran and the country's legal institutions are contrary to international law, without value and -- as in the past -- doomed to failure," he said.

But for the Bush administration, the "axis of evil" member foments global Islamic extremism, supports insurgents attacking US troops in Iraq, and is hell-bent on making nuclear weapons.

Bush suggested last week that a nuclear-armed Iran could trigger "World War III," while Vice President Dick Cheney spoke on Sunday of "serious consequences" unless the Islamic republic comes to heel.

Cheney's language matched the 2002 resolutions of the UN Security Council that warned Iraqi dictator Saddam Hussein of "serious consequences" unless he came clean on his alleged stockpiles of WMD.

Unveiling the new sanctions, US Secretary of State Condoleezza Rice renewed an offer to meet her Iranian counterpart for talks "anytime, anywhere" and on any issue if Tehran agrees to suspend its enrichment of uranium.

But Rice also warned: "If they choose to continue down a path of confrontation, the US will act with the international community to resist these threats of the Iranian regime."

Manochehr Dorraj, professor of international relations at Texas Christian University, said the sanctions' bite depends "in large measure" on cooperation from European allies such as France and Germany, and from Russia and China.

But Dorraj said that if Iran continues to enrich uranium, then the Bush administration can argue that sanctions have failed "and that the military option is the only viable alternative."

Senior US officials made clear their displeasure with what they called the stalling tactics of Russia and China as the Security Council prepares to debate Iran's nuclear program anew next month.

Russian President Vladimir Putin Thursday urged restraint over Iran following the latest US sanctions.

"You can run around like mad people wielding razor blades, but it is not the best way to resolve the problem," he said.

Iran has now come to dominate foreign-policy exchanges on the US presidential campaign, as leading Republican contenders and Democrat Hillary Clinton vie to adopt the toughest posture on its nuclear drive.

Professor Paul Pillar, an Iran expert at Georgetown University, said the Bush administration seems to be trying to close down its successor's options on Iran, as sanctions are much harder to lift than they are to impose.

The White House now has "a posture of confrontation" with Iran, and hardliners in both Washington and Tehran are "bringing out the worst in each other," he said.

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