As the Los Angeles Clippers and Boston Celtics seemingly inched closer to an agreement that would bring an end to their on-again, off-again negotiations and finally send both Celtics star forward Kevin Garnett and coach Doc Rivers to Los Angeles, sources close to the process said Thursday that a hurdle at league level threatens to scuttle the talks yet again.

Sources told ESPN.com that it has been communicated to both teams by the league office that the NBA has questions about the proposed transactions involving Garnett and Rivers and the appearance that they are connected.

The teams awoke Thursday, sources said, close to an agreement on separate transactions that would land Garnett and Rivers in Los Angeles. The first is a proposed one-for-one player swap that would send Clippers center DeAndre Jordan to Boston for Garnett, who would waive his no-trade clause to clinch the deal. The second move would require the Clippers to convey two first-round picks to Boston as compensation for the Celtics agreeing to let Rivers out of the three years and $21 million left on his contract.

League rules not only prohibit coaches from being formally "traded" but likewise prevent teams from making any trade or free-agent signing with contingencies or side deals attached. So it is incumbent on the Celtics and Clippers to convince the league office that these transactions would be made independently.

"I would say in the language of diplomacy that the teams are aware that the collective bargaining agreement doesn't authorize trades involving coaches contracts," NBA commissioner David Stern said on the "Stephen A. Smith Show" on ESPN Radio on Thursday. "The only consideration that can be done here in players transactions are other players, draft picks and a very limited amount of cash. But, coaches contracts don't qualify as extra consideration or acceptable consideration in player transactions. The teams know that ... it has been confirmed to them. What we wouldn't prohibit ... what the rules wouldn't prohibit ... Let me start again ... what the rules won't allow ... it can't be gotten around by breaking it up into two transactions."

One source with knowledge of the talks told ESPN.com on Thursday that the Celtics and Clippers have been negotiating for days knowing that league rules could well prohibit the transactions from going through.

ESPN.com reported Wednesday that the Clippers, despite taking an evening break from the discussions so owner Donald Sterling could meet face to face with Rivers' fellow coaching candidate Brian Shaw, had quickly made progress with the Celtics after re-opening talks and relenting on their previous unwillingness to surrender only one first-round pick to persuade Boston to let Rivers become a coaching free agent. League rules do allow teams to offer draft picks or cash to get a coach under contract released from those obligations.

After meeting with Shaw, Clippers officials met separately with Sterling late Wednesday to review all the candidates and options, sources told ESPN.com.

One source close to the process came away from Wednesday's developments expressing optimism the sides would reach an agreement in principle by Thursday on the two deals involving Garnett and Rivers, which the Clippers believe will all but cement the signature of star guard Chris Paul when Paul becomes an unrestricted free agent on July 1. Yet it remains to be seen, sources said, how big a hurdle this is and whether the teams will be able to get league approval on the current proposed framework of the two deals or be forced to propose alternatives.

The Clippers' Jordan, in an appearance on ESPN Radio's "Mike and Mike in the Morning" on Thursday, said he understands "it's a business" and wouldn't have any hard feelings for the team that drafted him if he's shipped out.