The leaders of Venezuela and Iran on Monday boasted that together they would be victorious over the United States, saying the fall of the U.S. dollar was a prelude to the end of American dominance in the world.

Venezuelan President Hugo Chavez paid a visit to his Iranian counterpart Mahmoud Ahmadinejad in a show of the strength of their friendship, bound by opposition to Washington. The visit came after the firebrand duo unsuccessfully tried over the weekend to push OPEC away trading in the slumping greenback.

The OPEC summit, in Riyadh, showed the limits of their alliance -- their proposals were overruled by other cartel members, led by Saudi Arabia. But it also showed their potential for stirring up problems for the U.S. and its allies.

The alliance between Chavez and Ahmadinejad has blossomed with several exchanged visits -- Monday's was Chavez's fourth time in Tehran this year -- a string of technical agreements and a torrent of rhetoric presenting their two countries as an example of how smaller countries can stand up to the superpower.

"Here are two brother countries, united like a single fist," Chavez said upon his arrival in Tehran, according to Venezuela's state-run Bolivarian News Agency.

"God willing, with the fall of the dollar, the deviant U.S. imperialism will fall as soon as possible too," Chavez said after a two-hour closed meeting with Ahmadinejad, the Iranian state news agency IRNA reported.

As the dollar weakens, oil prices have reached a record high of $100 per barrel. But Chavez warned over the weekend at the Organization of Petroleum Exporting Countries that prices would more than double to $200 if the U.S. attacks Iran or Venezuela.

"The U.S. empire is coming down," Chavez also told Venezuelan TV, calling the euro a better option and saying Latin American nations are also considering a common currency.

Leftist Chavez is a fierce enemy of U.S. President George W. Bush, and Iran's Islamic government is in a bitter standoff with Washington over Tehran's nuclear program. The United States accuses Iran of seeking to develop a nuclear weapon, a claim Tehran denies, and Iran has come under U.N. sanctions for refusing to suspend uranium enrichment.

Ahmadinejad backed his "dear brother" Chavez in their joint plight against the Bush administration.

"We have common viewpoints and we will stand by each other until we capture the high peaks. God is with us and victory is awaiting us," Ahmadinejad was quoted as saying by IRNA. He said he and Chavez would stick together to defend their "nations and ideals to the end."

During the weekend oil summit, Iran and Venezuela proposed that the Organization of Petroleum Exporting Countries begin pricing its oil in a basket of currencies, rather than just the dollar, and wanted the summit to specifically express concern over the dollar's slide in its final statement.

U.S. ally Saudi Arabia blocked the Iranian and Venezuelan moves -- though in an apparent compromise, the organization directed its finance ministers to study the issue of the dollar.

Chavez repeated his warnings that attacking Iran would further increase oil prices. "It's very important that they leave us in peace, the major oil-producing countries," he said.

"If it occurs to Bush to invade Iran, I'm sure the Iranians will resist, and they aren't going to allow them to take away their oil, just as we Venezuelans wouldn't allow it," Chavez said.

In Tehran, the two presidents signed four memorandums of understanding on Monday to create a joint bank, a fund, an oil industry technical training scheme and an industrial agreement, Iranian state television reported. It said Chavez then left Iran after an official farewell ceremony.

On the previous Chavez visit in July, the two countries broke ground on construction of a jointly owned petrochemical complex in Iran, with 51 percent of it in Iranian ownership and 49 percent to be owned by Venezuela. The two also began construction of a second petrochemical complex in Venezuela, at a total combined cost of US$1.4 billion (euro956 million).

The two nations believe their petrochemical partnership will help Iran access markets in Latin America and Venezuela would get access to energy markets in Asia, especially India.

Since 2001, the two countries have signed over 180 trade agreements, worth more than US$20 billion (euro15 billion) in potential investment, according to official reports.

Iran has partnered with Venezuela on several industrial projects in the South American nation, including the production of cars, tractors and plastic goods.