"The US government should know that if they cross the line they will not have any Venezuelan oil," Chavez said at a public event on Friday.

"I have started taking measures in that respect, I'm not going to say what."

Venezuela, the world's fifth-biggest oil exporter, supplies about 15% of US energy imports.

Chavez's statements came a day after Condoleezza Rice, the US secretary of state, said that the Venezuelan government posed "one of the biggest problems" in the region and that its ties to Cuba were "particularly dangerous" to democracy in Latin America.

Speaking to government supporters at the presidential palace, Chavez said "many countries ask us for more oil and we have had to tell many countries we can't send them more" because Venezuela - the world's fifth largest oil exporter - ships 1.5 million barrels of oil a day to the United States.

Relations between Chavez and the administration of George Bush, the US president, hit new lows in recent days after Washington expelled a high-ranking Venezuelan diplomat in response to Chavez booting out a US embassy official for alleged spying.

Chavez, a fierce Washington critic, accuses the US government of repeatedly trying to discredit his government and orchestrate his ouster. American officials deny those charges but accuse him of authoritarian tendencies and threatening democracies in the region.