By Marianna Parraga, Gary McWilliams and David French

HOUSTON (Reuters) - An oil refiner backed by activist investor Carl Icahn and a group of creditors holding claims against Venezuela are competing in the last mile of a U.S. court auction for Citgo Petroleum, according to three people familiar with the matter.

A total of 18 creditors holding rulings for debt defaults and expropriations in Venezuela totaling $21.3 billion are pursuing proceeds from the auction of shares in Citgo's parent, organized by a federal court in Delaware. But offers are not expected to cover that amount entirely.

An investment group that includes miner Gold Reserve and a unit of conglomerate Koch Industries have offered about $9 billion in combined cash and claims against Venezuela, two of the people, who were not authorized to speak publicly about the matter, said.

Icahn-controlled CVR Energy separately submitted an all-cash bid of about $8 billion, the people with knowledge of the auction said.

Both current offers are above the highest $7.3 billion bid submitted in a first round earlier this year, but far away from Citgo's market valuation of between $11 billion and $13 billion.

The court officer overseeing the auction, Robert Pincus, last week requested extra time to evaluate the complex offers and reach an agreement on terms. Creditors can use claims against Venezuela in lieu of cash in some cases.

The process has allowed companies including Gold Reserve, which has a more than $1 billion claim, and Koch Industries, with a $457 million claim, to join forces with investors advised by investment bank Centerview Partners, who have contributed cash to the bid, the two sources added. Creditors can apply claims as equivalent to cash.

Gold Reserve and court officer Robert Pincus declined to comment. Centerview did not have an immediate comment. Koch did not reply to requests for comment. CVR declined to comment, but the company's chief executive in late July told investors it was exploring strategic transactions in refining.

Gold Reserve in June said it had obtained $36 million through a private placement of shares and lined up a partner, FJ Management, to support its bid.

At least five groups of investors submitted binding bids in the second round, and three secured financing commitments from banks and advisors including JPMorgan, Morgan Stanley and Rothschild & Co, people close to the matter told Reuters in July.

According to the court's terms, bidders have an opportunity to add further parties to their offers to boost their value and can also top off their bids after submitted, making the process dynamic.