NEW YORK (AP) _ Resentment is building between on-field officials and owners threatening to lock them out if no agreement is reached before the season opens Sept. 9.

The league began hiring replacement officials Wednesday, paying each $4,000 even if the dispute is settled and they never work a game. Under the agreement, they are guaranteed $2,000 each for two games whether there is a strike or lockout or not.

An NFL source also confirmed that Ed Hochuli, one of the league's top referees and the president of the union, sent an e-mail to 1,200 potential replacement officials saying ``don't go down as one of the scabs who stabbed the NFL guys in the back.''

The e-mail, first disclosed by The Washington Post, went on to say: ``What may sound like a fun diversion, a fun couple of games for you, is my career.'' Hochuli, a Phoenix-based lawyer, added: ``Working as a scab will actually hurt and likely kill any chances you would have of ever getting into the NFL.''

A league source, speaking on condition of anonymity, said the NFL heard from a number of the potential replacements, who said they resented the tone and the implied threats.

The sides talked for four hours Monday with no progress.

The league has offered to double officials' pay. The union, which says its officials make one fifth of the salaries in baseball, basketball and hockey, wants salaries on a par with them.

``We were disappointed there was no counterproposal,'' league spokesman Greg Aiello said.

Tom Condon, the agent who represents the officials, has said at various time that he expects talks to come down to the final days, ``as they usually do.''