The National Basketball Association moved on the offensive yesterday against the players association, which has threatened a strike April 2 if a new collective bargaining agreement isn't reached by then.

The league said that it plans to take a number of actions against the players if they strike, the most drastic being to deny free agency to any striking player scheduled to become a free agent at the end of this season.

The NBA has sent a letter to Special Master Kingman Brewster, asking for confirmation that it is within its rights to take such an action.

"The NBA has commenced an enforcement proceeding against the players association to obtain a declaration confirming that any player who fails to complete his playing contract because of a strike, will not become a veteran free agent," said an NBA spokesman. "As a result, players who otherwise would be eligible to negotiate with new teams this summer will be prohibited from doing so."

About 50 players, including Kevin McHale of Boston and World B. Free of Golden State, are due to become free agents at the end of the season.

Larry Fleisher, general counsel for the players association, said the league "is wrong, just like it's been in all of its legal maneuverings. They (the NBA owners) are determined to use every conceivable threat to confuse the issue."

The league and the union have been without a labor agreement since June 1. Their last meeting, Tuesday in New York, lasted only 24 minutes with no progress.

No future meetings have been scheduled, but the NBA said yesterday it had received a letter from Fleisher asking for a resumption of negotiations and a response to that request was "being formulated."

The league said it is advising the players that if they strike, the following sanctions will be enforced:

No salaries will be paid; no deferred payments will be paid for the remainder of the year; all insurance, medical and health plans will be terminated; all incentive bonuses will be withheld; any player who has already been paid his full yearly salary will have to return a portion of it for the length of the strike; if any teams are forced out of business because of the strike, the league will not be responsible for any salaries owed players on those teams.

The two sides have basically agreed to a guaranteed compensation plan, which will put a ceiling on each team's payroll and give the players a fixed percentage of league revenues. The stumbling block is over when to implement the plan.

The owners want it implemented immediately, but the players don't want it until after the 1986-87 season, when the Oscar Robertson suit settlement expires. The Robertson settlement is responsible for the present free-agent system in the NBA.

"The owners are using scare tactics and it is those very tactics which could in the long run force a strike," Fleisher said.