By Edward Chaykovsky

If Amir Khan intends to reach an agreement with his domestic rival Kell Brook - he will need to do so in a hurry, because time is running out.

Brook (36-1, 25 KOs) was mandated long ago to make a mandatory defense of his IBF welterweight title against undefeated Errol Spence.

The sanctioning body allowed him to move up to the middleweight division last September, when he challenged WBA, WBC, IBF, IBO world champion Gennady Golovkin. Brook was stopped in five rounds and suffered a fractured orbital bone.

Brook had to undergo surgery and was allowed several weeks from the IBF to fully recover. He finally got back into training and wants to return to action before the first half of this year.

Khan (31-4, 19 KOs) has also been out of the ring with injury, since getting knocked out last May by Saul "Canelo" Alvarez. Khan underwent surgery to correct a lingering issue with his right hand.

Brook wants to fight him as soon as possible, but Khan wants to take an interim-fight in April.

Eddie Hearn of Matchroom Sport, who promote Brook, has received an additional extension from the IBF to begin negotiations with Spence.

At the moment, Spence is looking more and more likely as Brook's next opponent.

"Kell is back in training, but he's not able to spar yet, so we've requested a further four weeks from the IBF for negotiations [with Spence]," Brook's promoter, Eddie Hearn told scribe Nick Parkinson.

"We're speaking to Khan. It could be Khan, if he wants it, but we can't promise that will happen. He wants a warm-up fight, and we want to go straight into it. Our priority is to listen to the IBF on who we have to fight, and they are telling us that will be Spence. I don't want to annoy the IBF about it. In 28 days we will start negotiations. It [Brook's next fight] will happen in May, and in the UK I expect. We've got 75 percent of the purse bid to play with, so I can't see us losing that."