WASHINGTON (AP) _ Federal mediators moved Monday to help end a 2-day-old strike against American Telephone & Telegraph Co. by its largest union, amid evidence the two communications giants could not connect.

Kay McMurray, director of the Federal Mediation and Conciliation Service, met Monday with both sides, said Dennis Minshall, a spokesman for the agency.

''Hopefully, he will find a formula to help us end this strike,'' said Morton Bahr, president of the Communciations Workers of America, whose 155,000 member employees of AT&T struck the company at midnight Saturday. ''Our objective is to reach it well before the end of this week.''

Herb Linnen, a spokesman for AT&T, described Monday's talks with McMurray as informal. ''We are not talking about bargaining issues in the mediation at this point,'' he said. Both sides agreed informally to the help after being approached by the mediation service, Linnen said.

Bahr earlier Monday instructed the union's negotiators to approach AT&T bargainers about new company assurances that ''systems technicians'' cable pullers who now make $646 a week would not be laid off under the new contract and then rehired at a maximum pay of $360 per week.

However, no formal talks between the two sides were held Monday, officials for each said.

The union's belief that language in the company's final offer would allow downgrading between 15,000 and 20,000 of its systems technician service workers into lower-pay categories was the main disagreement that triggered the strike.

''We're not going to throw those people to the wolves,'' Bahr had said on Sunday.

AT&T officials said Sunday night and repeated Monday that no systems technicians now on the company's payroll would take a pay cut.

''If that's the position of this company, we can settle this issue,'' Bahr told AT&T's executive vice president, Charles Marshall, in a joint appearance by both men on ABC's ''Good Morning America'' program.

Labor Secretary William Brock had said earlier Monday that he doubted that the federal government would intervene in the strike.

''The sides have to settle their own difference,'' Brock said. ''We don't have any wisdom in government to tell one side they've got to do this or that.''

The Mediation and Conciliation Service's role would be largely an advisory one, explaining to each side the other's position. The agency seeks to resolve labor disputes when managers and representatives of workers cannot do so on their own. But the agency does not impose settlements.

Asked whether the dispute might be settled without federal mediation, Bahr said, ''We could, but sometimes you get so close to the situation that it takes a third party.''

AT&T has offered the CWA and another union, the International Brotherhood of Electrical Workers, nearly identical contracts with an 8 percent total wage increase over the next three years - 2 percent immediately and 3 percent in 1987 and again in 1988.

The IBEW on Sunday tenatively accepted the package on behalf of its nearly 41,000 member employees of AT&T subject to a ratification vote within the next two weeks.

However, the CWA said two provisions other than the one on systems technicians were unacceptable to it.

One was AT&T's demand for eliminating automatic cost-of-living adjustments to reflect inflation - an ingredient of every telephone workers contract since 1972. The other was the proposed elimination of incentive pay on some jobs performed by the 20,000 manufacturing workers.