The Federal Trade Commission yesterday temporarily barred Mobil Corp. from increasing its stake in U.S. Steel Corp., the company that bested Mobil in the bidding war for Marathon Oil Co.

The FTC told Mobil yesterday that it must submit more information before the agency decides whether the oil giant may increase its holdings in the steel company or whether such a step appears to violate federal antitrust laws.

Mobil filed a notification with the FTC on Dec. 8 that it might purchase up to 25 percent of U.S. Steel's stock--21.8 million shares. The FTC acted shortly before the expiration of a waiting period with a request for more information rather than a decision on whether to sue to block the proposed purchase.

Mobil's apparent point in increasing its holdings would be to try to wrest away from U.S. Steel control of the oil and gas reserves that the steel company will acquire with Marathon.

Since Mobil's defeat at U.S. Steel's hands earlier this week, industry analysts have said they consider it unlikely that Mobil actually would try to seize control of U.S. Steel. Such a measure probably would run into the same costly legal problems that have doomed Mobil's at tempted takeovers of Conoco and Marathon.

Unless the FTC decides to sue, Mobil may proceed with its purchase of U.S. Steel stock 20 days after it submits the information requested by the FTC. A Mobil spokesman said that the company is studying its options and has made no decision on what its next step will be, virtually the same comment the firm has made every day since U.S. Steel's victory became apparent.

That happened Wednesday, when U.S. Supreme Court Chief Justice Warren Burger denied Mobil's emergency application for an injunction against U.S. Steel's purchase of control of Marathon. At 12:01 a.m. Thursday, hours after Burger's decision and a second after a legal prohibition against U.S. Steel's purchase of Marathon stock expired, the steel producer said those purchases were beginning.