The United Mine Workers of America, which last week concluded its first strike-free nationwide contract in 20 years, is now the target of a strike by 350 employes, who have walked off the job at a UMW health and pension fund run jointly by the union and 32 coal companies.

The strikers, about 250 based here and 100 in coal-field branch offices, stopped work Monday in a dispute over concessions demanded by the company-union management. The fund, which serves some 185,000 active and retired miners, is controlled by five trustees appointed by the UMW and the Bituminous Coal Operators Association, a 32-firm group that employs most of the nation's coal miners.

"We are only asking for what's fair: the same things the UMW asked for" in its recently concluded negotiations, said Robert Olds, vice president of the independent union. Olds said that UMW insisted on a "no-concessions" contract in its bargaining with BCOA, but that both parties are now trying to impose concessions on fund employes.

Trustees are demanding a 2 1/2-hour increase in the 35-hour workweek without a pay raise, which the union equates with a 6.7 percent pay cut, Olds said. Management also is seeking to cut pension benefits by up to $100 a month for some lower-paid employes, while allowing raises for higher-paid, which the union strongly resists.

Olds said the UMW, which successfully bargained a new contract for more than 100,000 of its members, is hurting those same members by "forcing" a strike at the fund.

He said his union is strongly opposed to a new salary and job-classification system that he said would deny annual raises to many employes, and give 3 percent raises to others, while also imposing ceilings on pay once veteran employes reached maximum levels. The new system would also make it easier to fire employes without a grievance procedure, he said.

Spokesmen for the trustees and the UMW did not return calls yesterday.

The union, the UMWA Welfare and Retirement Fund Employees Union, is picketing the fund office at 2021 K St. NW, and eight field offices in Big Stone Gap, Va., West Virginia, Pennsylvania, Indiana and Kentucky.

The funds have continued operating during the strike with about 100 managerial employes processing payments and handling problems.