Administration spokesmen, speaking two days before the Saturday deadline for imposition of President Reagan's economic sanctions against Libya, said yesterday they have "no evidence or reason to believe" that U.S. oil companies have shifted their Libyan assets or operations to foreign subsidiaries or non-American producers.

However, other officials acknowledged that the administration is negotiating with the companies on finding ways to dispose of their Libyan holdings, whose total value has been estimated at $1 billion, in an orderly manner.

The objective, the officials said, is to spare the companies the undue financial loss that would result from abandoning their interests to Libyan leader Muammar Qaddafi's government, while adhering to Reagan's policy of causing the maximum possible damage to the Libyan economy.

In a related development, Defense Secretary Caspar W. Weinberger said yesterday that the United States would complete its carrier flight operations off the Libyan coast last night as scheduled. He also said that warships of the Sixth Fleet will remain in the central Mediterranean region for some time.

The ending of the week-long maneuvers, which U.S. officials had described as "a show of resolve" against Qaddafi's support for international terrorism, shifted attention to progress on implementing the total economic embargo announced by Reagan Jan. 7 in retaliation for Libya's alleged complicity in the Dec. 27 attacks by Palestinian terrorists at the Rome and Vienna airports.

Reagan ordered U.S. firms to end all trade and economic exchanges with Libya and called on the estimated 1,500 Americans in Libya to leave. However, with the approach of the Saturday deadline, U.S. officials, citing what State Department spokesman Bernard Kalb called "the complicated, difficult task of getting it all together," said yesterday that they were unable to give details or statistics about how Reagan's orders are being carried out.

Most of the confusion centered on the status of the five U.S. companies that account for a substantial part of Libyan crude oil production and sales. The Washington Post, in a dispatch from Tripoli, yesterday quoted diplomatic and industry sources there as saying that four of the companies -- Occidental, Marathon, Conoco and Amerada Hess -- have been undercutting the embargo by lending or selling their production to European companies under various reimbursement arrangements.

Kalb and White House spokesman Larry Speakes, using similar language, said the companies had assured the administration that they had not engaged in such practices. "We have no reason to believe they have taken any action inconsistent with the president's requests," Kalb added.

Conoco and Occidental also issued statements saying they have been in full compliance with Reagan's orders and have not exported nor transferred any Libyan oil since the sanctions were announced.

However, other U.S. officials, speaking on condition they not be identified, said that the Treasury Department is negotiating with the companies about how they can divest their Libyan operations in a way that one official described as "not costing them millions of dollars and not making Qaddafi a present of their equipment and concessions." The officials said that each company's legal position is different and, as a result, the negotiations are "exquisitely complicated."

The officials said that such a procedure almost certainly will involve the sale or transfer by other means of the American companies' assets. They also said that since the other western industrial countries have refused to impose sanctions against Libya, the vacuum left by the U.S. firms is expected to be filled by non-American producers.

The officials noted that senior administration officials have already conceded that the reluctance of Western Europe, Japan and Canada to join the United States in a boycott of Libyan oil means that the U.S. sanctions will have only limited effect.

But the officials said the administration is working on the assumption that such factors as the loss of American technicians, inability to obtain spare parts for American-made equipment and lack of access to American servicing companies will cause at least short-term inconvenience to Qaddafi's ability to produce and market the crude oil that is the source of Libya's wealth.

The officials said the resultant problems for Libya, coupled with the current world oil glut and mismanagement of the Libyan economy, should help reduce the revenues that Qaddafi has available to finance terrorist activities, showing him that his course of conduct is not cost-free.

The officials also said that their best evidence indicates that most of the Americans who have been working in Libya have left the country or are planning to do so. They added, though, that since there is no requirement for those Americans to report their departure, the administration does not have reliable figures on how many have left.