The United States has reached a preliminary deal to buy F-35 jets from Lockheed Martin worth about $13 billion, clearing the way for a larger multi-year purchase that aims to bring the cost per jet down to $80 million by 2020.

The deal for 141 F-35s lowers the price of the F-35A, the most common version of the stealthy fighter jet, to about $89 million, down around 6 percent from $94.3 million in the last deal struck in February 2017, the sources familiar with the talks said.

Bringing down the cost of the world's most expensive defence program is crucial to securing more orders, both in the United States and abroad.