* Iraq wants to buy up to 36 F-16s
* Fighter jet considered key to building Air Force (Adds updated Lockheed comment, paragraph 5)
By Suadad al-Salhy
BAGHDAD, Sept 26 (Reuters) - Iraq has signed a contract to buy 18 Lockheed Martin (LMT.N) F-16 warplanes to bolster its air force, an adviser to Prime Minister Nuri al-Maliki said on Monday.
The value of the deal was not immediately known, but a senior U.S. military official said recently the offer on the table for the Iraqi government was valued at "roughly $3 billion."
Iraqi and U.S. military officials have said strengthening its air force is one of Baghdad's top priorities as U.S. troops prepare to leave by Dec. 31, more than eight years after the invasion that ousted Saddam Hussein.
"The contract was signed ... and a part of the contract cost was sent to the bank account of the company," said Maliki's media adviser, Ali al-Moussawi.
Lockheed said in a statement it was "pleased by the announcement that the governments of Iraq and the United States" have agreed on the sale. The company added it welcomes Iraq "as the 26th nation to operate the F-16."
The Pentagon said it was aware of news reports that Iraq had signed the agreement but did not immediately confirm and referred questions to Baghdad.
Iraq has long sought a combat jet for its rebuilt air force. The government delayed a planned purchase of F-16s in February to divert a $900 million down payment to its national food ration programme to help quell street protests.
Maliki said on July 30 Iraq would buy 36 F-16s, double the number it had originally planned, to shore up its weak air defenses. The OPEC producer has found itself flush with cash this year, reaping windfall profits as world oil prices have remained above budget projections.
The two sides have been negotiating for the F-16 Block 52 export model with sophisticated avionics and weapons in a deal that included maintenance and training, a U.S. military official said.
Iraq is relying on the U.S. military for air support as it rebuilds its forces and battles a stubborn Islamist insurgency. Washington and Baghdad are discussing whether to keep some U.S. troops or military trainers in Iraq beyond the year-end deadline for U.S. departure.
Iraq's two airborne defense units, the Air Force and Army Aviation Command, have only 158 aircraft, including 89 helicopters and 69 airplanes, and about 7,500 personnel, according to U.S. figures.
At present, it has three Cessna Caravan propellor planes equipped with Hellfire missiles but no combat jets, a U.S. military official said. (Additional reporting and writing by Jim Loney; Editing by Tim Pearce and Todd Eastham)