Venezuelan President Hugo Chavez has set the world's biggest oil companies a May Day deadline to surrender control of multibillion-dollar crude projects.
Oil Minister Rafael Ramirez said the state would seize the fields if the deadline were missed, threatening firms such as Chevron, Exxon Mobil, Conoco Phillips, Statoil and BP.
Mr Chavez started this countdown a day after he received 18-month powers to rule by decree, enabling him to speed the centralisation of economic and political power in his self-styled leftist revolution.
"I have given instructions that on May 1st - May 1st - all the fields of the Orinoco Belt should wake up under our control," he told a news conference.
In Washington, the White House said it hoped US companies would be treated in accordance with international regulations.
The US Department of Energy condemned the nationalisations as "a disturbing trend away from open and transparent market principles".
Mr Chavez reiterated Venezuela would take a stake of at least 60 per cent in four projects developing the crude of eastern Venezuela's Orinoco Belt into synthetic oil. He hoped the foreigners would stay on as minority partners.