Citigroup's sale of OneMain Financial faces rigorous regulator scrutiny. Reuters

Citigroup's plan to divest its subprime lending unit OneMain Financial may be delayed due to potential antitrust concerns.

Springleaf Holdings, which is set to acquire the unit, said the deal would be delayed beyond the third quarter, as US regulators including the Department of Justice (DOJ) have not yet completed their review of the proposed acquisition.

"The DOJ and certain state Attorneys General have expressed to Springleaf potential concerns with respect to the proposed acquisition," the company said in its earnings release.

Springleaf said it agreed to postpone the completion of the deal scheduled for 10 September in order to give the department enough time to finish its antitrust review.

It added that it expected talks with the DOJ and certain state attorneys general to be constructive.

Citi earlier announced the sale of OneMain Financial Holdings to Springleaf Holdings for $4.25bn (£2.7bn, €3.9bn) in cash, saying the business did not fit its strategy.

The bank has been seeking to sell off OneMain since at least 2011 as part of its wider plan to divest non-core assets and focus on wealthier clients.

For the second quarter ended on 30 June, Springleaf reported pretax core earnings of $107m, compared to $94m in the same quarter of 2014.