Four U.S. oil firms that pump the bulk of Libya's oil and a big American construction firm that is helping to irrigate the Libyan desert are targets of the sanctions announced by President Reagan last night, according to administration officials.

The new sanctions, the latest in a series of anti-Libyan measures dating to 1981, are intended to stop all American economic activity with Libya and bring all Americans home, officials said. The actions, they said, were necessary to make the regime of Col. Muammar Qaddafi pay a price for its support of international terrorism and render it "a pariah" among nations.

The effectiveness of the new measures will depend largely on whether European nations, by far Libya's most important trading partners, follow suit, according to the officials who briefed reporters at the White House before President Reagan's news conference.

The administration acted before asking the Europeans to cooperate on sanctions against Libya, an official said, but added that "we are now in a much better position, having taken this step, to call upon them for concerted action."

Reagan last night acknowledged the difficulties of getting unified action against Libya. The United States will consult its European allies on punitive measures, the president said, but added, "I don't know that we're going to outright ask them . . . Some of them may have problems of their own in their own economies that are going to . . . render this nearly impossible."

Four U.S. oil companies -- Occidental Petroleum, Conoco, Marathon and Amerada Hess -- are now the main operators of Libyan oil fields and account for an estimated three-fourths of the country's total production of 1.1 million barrels per day.

Moreover, U.S. firms market at least one-third of Libya's production to customers in Europe and elsewhere, according to G. Henry M. Schuler, a former U.S. diplomat and oil executive in Libya who is now at Georgetown University's Center for Strategic and International Studies (CSIS).

Perhaps more important than the oil work to Qaddafi personally is crucial American participation, through the giant Texas construction firm of Brown and Root, in an ambitious $20 billion scheme to tap water far beneath the Libyan desert and transport it 1,200 miles to Tripoli and other populous areas.

Brown and Root's participation as project manager has been channeled through the firm's British subsidiary, which may be out of reach of Reagan's order. Both company and administration officials said it was not immediately clear whether, or to what extent, this work would be affected.

Price Bros. Co. of Dayton, Ohio, is providing the technology, machinery and equipment to manufacture concrete pipe for the project. Administration officials said any direct involvement by U.S. firms would have to stop by Feb. 1.

Qaddafi has called the irrigation project his "great dream" and "the eighth wonder of the world" and has continued to give it a high priority despite economic setbacks in recent years.

The measures announced last night are the latest in a series of administration actions over the past five years aimed at isolating and undermining Qaddafi. In 1981 and 1982, the United States banned importation of Libyan oil, then costing $5 billion annually, and asked all Americans to leave the country.

The oil stopped flowing to U.S. shores but 1,000 to 1,500 Americans remain, potential hostages of Qaddafi in the event of any future U.S. military action against Libya or other action that rankles the Libyan leader.

Specialists on Libya and the Mideast said last night that the new sanctions could seriously hurt the Libyan economy, unless Europeans or others come forward to fill the American role in Libya's all-important oil industry. Reagan warned that "Americans will not understand other nations moving into Libya to take commercial advantage of our departure."

But the experts expressed doubt over whether any economic measures would modify Qaddafi's support of terrorism.

How the often-mercurial Qaddafi will react to the new pressures was uncertain last night. Administration officials told reporters that "we hold Qaddafi accountable" for the safety of the Americans currently in Libya and said they expect him to allow Americans to leave without harrassment.

"I don't think this will have any effect on Col. Qaddafi's behavior," said Geoffrey Kemp, a former National Security Council specialist on the Middle East, now at CSIS. Kemp said the measures might make Qaddafi "more vehement" in his involvement with terrorists, but that Reagan had "no other option" than to take these steps.

Kemp added that by pressuring the Americans in Libya to leave, Reagan is "clearing the decks" for stronger action. "It makes it easier for us to do something unilaterally."

A Treasury Department official said a recent Supreme Court decision in a case involving travel restrictions to Cuba supports the administration view that it has the legal authority to regulate the financial transactions of U.S. citizens and corporations in a foreign country.

Because all transactions with Libya are banned under Reagan's order, the official said, Americans who buy or sell anything in Libya could be prosecuted for violating the order upon returning home. Violations would be a felony punishable by up to 10 years imprisonment and a fine, reporters were told.

Neither Reagan nor officials who briefed reporters ruled out the possibility of military action against Libya later. "If these steps do not end Qaddafi's terrorism, I promise you that further steps will be taken," the president said without elaboration.

Sen. David F. Durenberger (R-Minn.), chairman of the Select Committee on Intelligence, said Reagan's actions would help to make "a pariah state" of Libya. "We're starting to move on the countries that sponsor terrorism. Qaddafi is the first and there could be others," he said.

The legal basis for Reagan's sanctions was a determination that Libya's policies and actions "constitute an unusual and extraordinary threat to the national security and foreign policy of the United States" and a declaration of "a national emergency" to deal with it.

The actions were precipitated by the Dec. 27 terrorist raids in the Rome and Vienna airports attributed to a Palestinian group headed by Abu Nidal. Administration officials charged that Abu Nidal is "based in Libya" and "receives financial support as well as terrorist training in that country."