NBA referees believe that a lockout is both imminent and unavoidable after the latest break down in negotiations.

The referees rejected the league's latest offer by a unanimous vote on Wednesday. The two sides exchanged counterproposals on Thursday, but the talks have ended once again.

Lead union negotiator Lamell McMorris told ESPN's Chris Sheridan that the main issue holding up negotiations is a dispute relating to retirement benefits. The issue, severance pay, led NBA commissioner David Stern to end a formal bargaining session last week.

"It looks like a lockout is both imminent and unavoidable," McMorris said. "We have suspended dialogue again today. We've been in constant communication, but it's not going to happen."

"I am not optimistic," lead NBA negotiator Rich Buchanan said. "Based on what happened today, I'm surprised and disappointed."