(Reuters) - Boeing Co <BA.N> defended dealings with Iran on Thursday as further details emerged of a draft deal to sell or lease 109 aircraft to the country's main carrier.

Two senior Republican House of Representatives lawmakers said on Friday they were concerned that Boeing's plans to sell aircraft to Iran could threaten U.S. national security.

Replying to Jeb Hensarling and Peter Roskam, a senior Boeing executive said the Obama administration had told it that implementation of last year's deal to curb Iran's nuclear activities in exchange for lifting most sanctions was "critical to the national security interest of the United States".

A "memorandum of agreement" (MOA) calls for IranAir to buy a total of 80 aircraft from Boeing and lease a further 29 with Boeing's support, according to the letter, seen by Reuters.

Deliveries of the purchased jets are scheduled to start in 2017 and run through 2025.

"Boeing negotiated the MOA under the authority of the U.S. government following its determination that Iran had met its obligations under the (nuclear agreement)," wrote Tim Keating, Boeing's senior vice president of government operations.

The aircraft to be sold directly include 34 wide-body jets: 15 each of the 777-300ER and 777-9 and four of the 747-8 models, one person familiar with the deal said.

The proposed sale also includes 46 narrow-body jets: 40 of the upcoming 737 MAX model and six of the current 737NG.

Under the same provisional deal, Boeing will arrange for IranAir to acquire a further 29 737NG aircraft through leases.

Boeing <BA.N> declined to comment.

IranAir was not immediately available for comment.

The breakdown offers a boost to two current models that have seen declines in production lately: the twin-engined 777-300ER and four-engined 747-8, the latest version of Boeing's jumbo.

"This will help, but not solve, 777/747 production gaps," Leeham aerospace analyst Scott Hamilton wrote on Twitter.

Airbus agreed earlier this year to supply 118 jets worth $27 billion. Both planemakers must get U.S. export licenses due to key U.S. components in both portfolios of jets.

Boeing's letter said U.S. and European governments had indicated during nuclear talks that it was "key and essential" that Iran should be offered both U.S. and European aircraft.

People close to the talks have said that due to that complex diplomatic backdrop, the two deals could stand or fall together: contrasting with the "winner-takes-all" mentality of traditional clashes between the world's largest planemakers.

(Reporting by Tim Hepher and Parisa Hafezi; Editing by Alexandra Hudson and Sandra Maler)