Negotiators for the United Mine Workers and the coal industry agreed yesterday under federal pressure to "concentrated and continous" bargaining in a last-ditch attempt to avert a nationwide coal strike Dec. 6.

But no one was expressing optimism as the Federal Mediation and Conciliation Service took charge of the talks, meeting separately with each side in hopes of creating a climate for resumption of face-to-face negotiations.

Asked if a strike could still be averted, Wayne L. Horvitz, chief of the mediation service, said: "I don't know the answer to that. Mediators always have to believe you can reach a settlement without a strike."

The only chance of avoiding a strike appears to be a contract extension, which UMW President Arnold Miller has said he would consider only if the talks were making "substantial" progress. Asked Yesterday if an extension is possible, Miller said, "I kinda doubt it."

The UMW's contract with the Bituminous Coal Operators Association, whose 130 companies and 130,000 miners produce about half of the nation's coal tonnage, expires at 12:01 a.m. Dec. 6. But ratification of any agreement would take about 10 days, meaning a walkout is virtually assured without a contract extension.

Although government and industry sources have indicated a strike would not begin to deplete coal stockpiles for three months or more, Horvitz said he tried to impress on both negotiating teams the "extraordinary interest the government has in a quick and good solution to negotiations." along with a long-term improvement in the industry's bargaining relationships.

Federal mediators have been in contact with both sides from the start, but formally took charge of the talks after they broke off Friday with no plans for resumption.

After Hovitz met separately with two bargaining teams yesterday morning, other mediators met with the BCOA negotiators while Miller briefed his bargaining council and the union's executive board on the talks. The mediators were scheduled to meet with the union negotiators today.

As of late yesterday, plans called for the mediators to shuttle between floors of the Capitol Hilton Hotel, where bargaining teams have their negotiating offices, an exercise that one participant dubbed "elevator diplomacy." There was no sign of when face-to-face talks might resume.

After the two UMW meetings, union officials indicated little cause for hope. "We'll stay out till hell freezes over before we'll comply with their bottom-line proposal," said Frank R. Clements, president of UMW District 20 in Birmingham, Ala., referring to proposed punitive action against minors for wildcat strikes.

One of the key roadblocks in the negotiations is the operators' demand for greater stability in production and the union's demand for a limited right to strike over local issues , a clash that has proved irreconcilable thus far. The union also is demanding guaranteed financing of its medical and pension benefits, which the companies say cannot be done without production guarantees.