Caracas, Venezuela (CNN) -- Iranian President Mahmoud Ahmadinejad and Venezuelan President Hugo Chavez pledged closer cooperation on Monday, vowing to fight poverty and imperialism.

The two leaders signed a series of accords in the areas of industry, science, technology and politics, the state-run AVN news agency reported. Sitting side by side in front of a room full of reporters, they presented a united front in their opposition to the United States.

"It's clear they are afraid of our development," said Ahmadinejad.

Earlier, at a welcoming ceremony outside the Venezuelan presidential palace, the Iranian president called Chavez his "dear brother," and said his nation has peaceful intentions.

"Our weapon is logic. Our weapon is culture. Our weapons are human values," he said.

Chavez pledged to work with his Iranian counterpart to stop the "imperial insanity" of the United States, which he described as a "threat for the world."

The Venezuelan president mocked concerns from Washington about his country's relationship with Iran, joking that U.S. media reports made it sound as though he and Ahmadinejad would be launching missiles and bombs at the U.S. capital.

"We are not warmongers," Chavez said, as the Iranian president nodded in agreement. "Iran has not invaded anybody."

It's no coincidence that Venezuela is Ahmadinejad's first stop on a four-nation Latin America tour this week. Despite their cultural differences, Venezuela and Iran have found significant common ground: Both are among the world's top crude oil exporters, and their leaders are strong allies united by a fierce opposition to what they describe as U.S. imperialism.

The Iranian leader arrived in Caracas on Sunday and is traveling with his ministers for foreign affairs, energy and economy, according to IRNA, Iran's official news agency.

Over the years, the two nations have signed more than 270 accords, including trade deals and agreements on construction projects, car and tractor factories, energy initiatives and banking programs.

Chavez has also helped the Islamic republic forge relationships with other members of the Bolivarian Alliance of the Americas, an eight-nation regional bloc Chavez founded.

Even before he arrived, Ahmadinejad's plans for a Latin American tour grabbed global attention, as tensions grow between many Western powers and Iran over Tehran's nuclear program.

Officials in the United States and other Western nations have ratcheted up sanctions against Iran several times since a November report by the U.N. nuclear watchdog agency said the Iranian government was developing the technology needed to build a nuclear weapon. Last month, U.S. President Barack Obama announced sanctions against Iran's central bank.

On Monday the United Nations' nuclear watchdog agency confirmed that uranium enrichment has begun at a nuclear facility in northern Iran.

Iran says there's a medical purpose behind the nuclear program.

But in Washington, U.S. State Department spokesman Victoria Nuland said the confirmation that Iran was enriching uranium to "a level that's inappropriate" is "obviously a problem."

Not everyone in Venezuela is happy with Ahmadinejad's visit.

Diego Arria, an opposition politician, described the trip as a "provocation" to the United States and an embarrassment for Venezuela.

"It comes at the worst moment -- at a time when Ahmadinejad is being looked at by the international community with great fear.... It's bringing the threat to the United States closer to home," he said.

The Iranian president is scheduled to visit Nicaragua, Cuba and Ecuador later this week as part of his five-day tour.