The United States is "very close" to sealing a deal to sell 18 F-16 fighter jets to Iraq, a top Pentagon official announced Wednesday.

Iraq had frozen the $4.2 billion deal earlier this year amid the Arab Spring pro-democracy uprisings, but the prospects were "promising" for the contract to now move ahead, said General Russ Handy, head of the US air forces in Iraq.

"They are seeking to buy a larger number of F-16s (than) they had originally, up to 36," he told reporters.

"This first letter of offer and acceptance is for 18 of them, so we hope to hear very soon, but no final word yet on that."

The US military mission in Iraq is winding down ahead of a complete pullout of American troops by the end of the year but Washington is keen to sell as much equipment to Baghdad's fledgling security forces as possible.

"Everyone I talk to, everyone within the government in Iraq is convinced it's the right choice for them, so we are very encouraged by those words and we feel that we are very close to them signing that letter of offer and acceptance," said Handy.

The news comes after US Secretary of the Air Force Michael Donley visited earlier this month Iraq for talks on the F-16 sale after Prime Minister Nuri al-Maliki said on July 30 that he had revived the talks.

Iraq was looking at the purchase of 36 jets, Maliki said at the time, rather than the previously mooted 18.

Any deal would be worth billions of dollars and take years to implement, as it would require the manufacture of the aircraft and the training of Iraqi pilots. The F-16 has been exported to more than 20 countries.

Ten Iraqi pilots are already training on the fighter jet in the United States and their instruction could be completed before the delivery of the first planes to Baghdad, the general said.

In July, The Wall Street Journal reported that Iraq would also buy air defense systems from the United States.

US and local military officials have said that while they believe Iraq's forces to be capable of maintaining internal security, the country is lacking in terms of defending its borders, airspace and territorial waters.

American military officials continue to regard Iran as a destabilizing force.

"Iran does contribute to the violence we're seeing in the region and Iranian-backed militia groups are really the primary source of violence against both US forces and Iraqi forces," Handy said.

"These groups... clearly are receiving some backing from Iran. There are those in Iran who would like to see the government of Iraq weakened and not strong."