JPMorgan Chase is rethinking its purchase of RBS Sempra and could pick up only the joint venture's oil operations and all the non-U.S. businesses, a source familiar with the matter said.

But the U.S. power and gas businesses might be sold to Sempra Energy , the source said, which jointly owns RBS Sempra with Royal Bank of Scotland .

JP Morgan has been in exclusive talks to buy all of RBS Sempra for about $4 billion, but U.S. plans to stop proprietary trading by banks have prompted it to rethink its plans.

The plans by U.S. President Barack Obama should not derail the deal to buy RBS Sempra's oil operations and all businesses outside the United States, several sources have said.

The European Union is forcing RBS to sell its 51 percent stake in RBS Sempra. Sempra Energy had been in talks to sell its 49 percent as part of the deal, but may now take full control of the U.S. power and gas assets.

An RBS spokesman said constructive discussions regarding the sale of RBS Sempra were still under way.

JPMorgan declined to comment.

(Reporting by Luke Pachymuthu and Steve Slater; Editing by Hans Peters)