LONDON, Jan 30 (Reuters) - U.S. private equity firm Carlyle has begun exploratory talks with UAE investors to bring in partners should its initial agreement to buy Russian firm Lukoil's international assets proceed, three sources with knowledge of the process said.

Carlyle and Lukoil on Thursday ‌announced a preliminary deal that would transfer a wide range of assets - including oilfields in Iraq and refineries in ​eastern Europe - to the U.S. firm, pending approval from U.S. authorities that have placed ‍the Russian producer under sanctions. 

Neither company disclosed a valuation for ⁠the deal, which excludes ⁠Lukoil's Kazakh assets, because they have not yet agreed one, a separate source said on Thursday. 

The three sources said ‌state-controlled Abu Dhabi investors Mubadala, XRG and ​IHC had held talks with Carlyle about taking stakes in the Lukoil portfolio if the U.S. firm completes the purchase, though no deals have ⁠been reached.

A fourth source said the assets ‍were valued ​at around $20 billion. A fifth said the UAE investors were particularly interested in Lukoil's trading arm Litasco.

It remains unclear when Carlyle might bring in partners if ‍the transaction proceeds. The U.S. firm intends to keep the portfolio intact, one of the sources said. Private equity buyers in the energy sector typically hold assets for around five years before seeking to sell them on at a profit.

Lukoil, Mubadala and XRG did not respond to requests for comment.

Carlyle declined to comment. IHC said it ​would disclose ‍any material developments in accordance with applicable market regulations and requirements and that any announcements would be made in due course as required.

Carlyle said on ​Thursday it still needed to complete due diligence on the Lukoil assets. The deal would be structured in line with rules set by the Office of Foreign Assets Control (OFAC), the U.S. sanctions authority.

Lukoil has said it remains in talks with other potential buyers.

OFAC has said on its website that cash from any sale would have to be placed in an account under U.S. jurisdiction, ​with funds frozen until sanctions on Lukoil are lifted.

The U.S. Treasury has given Lukoil until February 28 to sell its global portfolio, which has drawn interest from several potential bidders.