Venezuela's ambassador to the U.S. has asked Sen. Richard Lugar to press Exxon Mobil Corp. to drop legal measures that have frozen $12 billion in assets worldwide.

"Through tactics that can only be compared with the very discredited strategy of pre-emptive war, Exxon Mobil has clearly violated the terms of the arbitration process," Bernardo Alvarez, Venezuela's ambassador to the U.S., said in a letter to the Indiana Republican that was released by the embassy Friday. Lugar is the top Republican on the Senate Foreign Relations Committee.

At issue are court orders that Exxon Mobil, the world's largest oil company, obtained in four countries freezing more than $12 billion in Venezuelan bank accounts and properties. The orders were part of Exxon Mobil's effort to get compensation in arbitration for last year's seizure of a Venezuelan oil field by the government of President Hugo Chavez.

Alvarez described Exxon Mobil's demand for $12 billion "clearly absurd," given the size of the company's investment in the disputed Cerro Negro oil project. In private discussions prior to the court orders, Exxon Mobil's demand never exceeded $5 billion, the ambassador said.

"You must understand that the actions recently and abruptly taken by Exxon Mobil against Venezuela's state-owned oil company might have a negative effect on global oil trade," Alvarez said in the letter, which was copied to other members on the Senate Foreign Relations Committee.

Chavez has threatened to cut off oil supplies to the U.S. and PDVSA has said it won't sell crude and refined products to Exxon Mobil.

Oil and Energy Minister Rafael Ramirez called the dispute an "economic war" with the U.S. and urged Venezuelans to march to protest the court orders that freeze the assets.

Oil workers and Chavez supporters held a vigil at the company's headquarters near downtown Caracas Thursday night.