Pentagon says it's unsure how AT&T shift affects it

No one knows yet - for sure - whether the breakup of the American Telephone & Telegraph Company endangers national security, a Pentagon official said Monday. The US Defense Department says an intense review of the antitrust settlement decree is under way.

Legislation may be necessary to ease the Pentagon's concern that the settlement breaking telecommunications giant AT&T into 23 separate companies might jeopardize military communications in an emergency.

His remarks seemed at odds with those of Assistant Attorney General William Baxter, who said Friday when the settlement was announced, ''I understand that the Defense Department has no difficulties with the arrangement we have worked out.''

Meanwhile, AT&T stockholders smiled while share prices soared Monday in both US and overseas trading.

AT&T stock, whose trading was halted on the New York Stock Exchange Friday at 587/8, opened Monday afternoon at 605/8, up 13/4.