A federal judge yesterday sternly ordered the Justice Department and American Telephone & Telegraph Co. to come up with a settlement of the government's antitrust suit against the Bell System by March 2 or go to trial two days later.

U.S. District Judge Harold Greene, responding to letters submitted Thursday by the government and AT&T requesting a delay in the massive antitrust suit, noted that the parties in the case had pledged that they would secure a decision by the Reagan administration on the direction of the case by Monday.

"To put it bluntly, the reliability of the parties' current commitments is inevitably impaired by the inconsistency of their present request with representations they made just two weeks ago," Greene wrote.

"Beyond that, the court is not prepared, on any basis, to countenance postponements, based on new and ever-changing problems and contingencies."

Greene announced his decision two weeks after completion of opening arguments in the six-year-old suit. The government is seeking to split AT&T into several units.

At the close of the opening arguments, however, Greene announced that he would delay the taking of evidence in the case in light of last-minute pleas by both the government and AT&T. At the time, Greene said in another memo that he would give the two sides until Feb. 2 to assert that the settlement of the case could be assured by March 4.

In addition, Greene said the government and AT&T had claimed they could "secure the approval of the incoming Justice Department within the next two weeks."

An AT&T spokesman said the company "will commit the necessary resources to do everything we can to reach a fair settlement by the March 2 deadline."