WASHINGTON (Reuters) - The federal mediator supervising the labor negotiations between the NFL and the players' union said Thursday "some progress" had been made in the week-long talks but that "very strong differences remain."

Federal Mediation and Conciliation Service director George Cohen said that talks would continue from March 1.

The current collective bargaining agreement between the league and the union (NFLPA) expires on March 4 and the threat of a lockout is looming.

The talks have focused on salary structures, benefits, plans to expand the regular season by two games and a number of other issues.

"Our time together has been devoted to establishing an atmosphere conducive to meaningful negotiations and, of course, matters of process and substance," said Cohen in a statement.

"I can report that throughout this extensive period the parties engaged in highly focused, constructive dialogue concerning a host of issues covering both economics and player-related conditions."

NFL Commissioner Roger Goodell has expressed fears of a fan backlash if the season is disrupted following failure to reach a deal.

The NFL is estimated to have an annual revenue of $9 billion and there are fears that sponsors may be frightened off by a lengthy dispute.

Cohen, who last year brokered a similar labor deal for Major League Soccer, said the tone of the talks had been respectful.

"The tenor of the across-the-table discussions reflected a noteworthy level of mutual respect even in the face of strongly held competing positions," he said.

"At bottom, some progress was made, but very strong differences remain on the all-important core issues that separate the parties. Nonetheless, I recommended and the parties have agreed to resume the mediation process in my office commencing next Tuesday.

"During the intervening weekend, the parties have been asked by us to assess their current positions on those outstanding issues," he added.

As part of the mediation process, which is non-binding, both the NFL and the union have agreed not to comment on the talks.

(Reporting by Simon Evans in Miami; Editing by Steve Ginsburg)