(Reuters) - Boeing Co <BA.N> is close to clinching a large sale of widebody jetliners to Qatar Airways, a deal that moved closer with the recent U.S. approval of Boeing fighter jet sales to the Gulf nation, according to two people familiar with the situation.

Qatar Airways is in the process of firming up the order, which it was due to have placed at July's Farnborough airshow, but which was delayed by the Qatari government in an effort to speed up U.S. approval for the fighter contract, one of the sources said.

Bloomberg reported that the deal was for at least 30 Boeing 777 and 787 jets, valued at about $6.7 billion at list prices.

Boeing declined to comment and Qatar Airways was not immediately available to comment.

If the deal is finalized, it would give the Chicago-based planemaker a substantial sales boost amid a slump in widebody orders this year.

It follows news on Wednesday that the United States had begun notifying lawmakers informally that it had approved the sales of 36 Boeing F-15 fighter jets to Qatar, valued at around $4 billion.

The United States also approved the sale of fighters to Kuwait. The sales to both states had been pending for more than two years amid concerns raised by Israel, Washington's closest Middle East ally, that they could be used against it, and criticism of Qatar for alleged ties to armed Islamist groups.

Qatar Airways Chief Executive Officer Akbar Al Baker said in August that the airline planned to make a major aircraft order to enable its network expansion plans.

In July the airline also said it was in advanced talks to purchase up to 30 of Boeing's 737 narrowbody planes.

Earlier in the year, Qatar Airways refused to accept three Airbus <AIR.PA> A320neo narrowbody jets, because of engine cooling issues. The airline has canceled its first A320neo delivery and said it must source alternate planes to meet demand.

Engine maker Pratt & Whitney <UTX.N> said in March it expected the cooling problem and other software issues to be fixed this year.

Airbus is struggling to keep up with deliveries to Qatar Airways of the long-haul Airbus A350 planes. Those delays, combined with the A320neo problems, has put Qatar Airways at risk of posting a loss in the fiscal year that ends in March, Al Baker said in August.

"Our relationship is very strained," Al Baker said in August. "What's happening at Airbus with the deliveries is seriously affecting our growth."

(Reporting by Ankit Ajmera in Bengaluru; Additional reporting by Alwyn Scott and Tim Hepher; Editing by Anil D'Silva and Bill Rigby)