Venezuela's President Hugo Chavez on Monday said America's suspension of arms sales to his country "doesn't matter to us at all."
Chavez, who is on a visit to London, also said his government would not respond immediately with punitive measures.
"It's the empire and it has a great capacity to do harm to the countries of the world," he said, referring to the U.S. as "an irrational empire."
The comments came after the U.S. State Department announced a ban on arms sales to Venezuela because of what it claims is a lack of support by President Chavez's leftist government for counterterrorism efforts.
Ealier Monday, speaking at a press conference alongside the mayor of London Ken Liningstone, Chavez called President Bush a genocidal "assassin" who was the "worst criminal in humanity."
Chavez, who is on a two-day visit to London aimed at energizing Europe's social movements, also repeated a warning that any military strike against Iran would send the price of crude oil soaring above US$100 a barrel. The current rate is around US$70 a barrel.
The Venezuelan leader said that such an attack on Iran would not prompt Venezuela to reduce its oil deliveries to the United States.
The world should do everything it can to avoid such a conflict, he said, adding that "Europe has an important role to play."
Chavez said he does not believe that Iran's nuclear program is a front for secret efforts to produce an atomic bomb.
"I don't believe that the United States or anyone else has the right ... to prohibit that a country has nuclear energy," Chavez said. "How many countries in the world have nuclear energy? Unfortunately, Venezuela doesn't have it."
Chavez also announced a plan to send heating oil to poor people in London for free or at discount prices. He said that the Venezuelan government should meet with the representatives of two refineries in Britain.
Later Monday, Chavez was to dine with guests including Harold Pinter, the Nobel Prize winning playwright, and actress and activist Bianca Jagger, and later give a lecture at an institute promoting cultural and commercial ties between Britain and Latin America.
However, he is not scheduled to meet with Prime Minister Tony Blair or any senior British government officials.
Relations between the two governments have soured since February, when Blair said in Parliament that Venezuela "should abide by the rules of the international community" and that he would like to see Venezuela's close ally Cuba become a "functioning democracy."
Chavez has called Blair a "pawn of imperialism" for his alliance with Bush.
The AP and The Times contributed to this report.