The United States announced on Thursday that it is imposing new sanctions against Iran, charging that Tehran supports terrorism in the Middle East, exports missiles and is engaging in a nuclear buildup.

The sanctions will be imposed against Iran's defense ministry, its Revolutionary Guard Corps and more than 20 Iranian companies, banks and individuals.

Under U.S. laws, any assets found in the United States belonging to the designated groups must be frozen. Americans are also forbidden from doing business with them.

The sanctions are believed to be the toughest Washington has levied against Iran since the 1979 takeover of the U.S. Embassy in Tehran. Secretary of State Condoleeza Rice, who was joined at a State Department news conference by Treasury Secretary Henry Paulson, described the U.S. sanctions as "a comprehensive policy to confront the threatening behavior of the Iranians."

They will "provide a powerful deterrent" for companies in the United States and abroad to sever business relationships with Iran, she said.

On Wednesday, Rice told Congress that Iran poses "greatest challenge" to American security, its interests in the Middle East and around the world.

The accusation and latest sanctions came days after U.S. President George W. Bush warned that a nuclear-armed Iran evoked the threat of "World War III," and Vice President Dick Cheney attacked the Islamic republic's nuclear drive.

Despite the tougher action against Iran, Rice insisted that the United States remains open to "a diplomatic solution."

Tensions are growing between the United States and Iran over Washington's accusations that Iran is secretly developing nuclear weapons and helping Shiite militias in Iraq that target U.S. troops. Iran denies the charges. 

Editor: Yan