WASHINGTON (REUTERS) - The United States (US) Defence Department on Wednesday said it has approved the sale of either the Lockheed Martin Corp F-35 stealth fighter or Boeing Co's F-15 Silent Eagle fighter to South Korea, which is expected to announce the winner of a 60-jet competition later this year.

The Defence Security Cooperation Agency, which oversees foreign military sales, notified US lawmakers last Friday about the possible sales to South Korea as tensions continued to mount with North Korea, saying that US warplanes would help Seoul "deter aggression in the region".

It issued two separate announcements on Wednesday, saying it had notified Congress about the possible sales of the two competing fighter jets, as well as radars, electronic warfare systems and other equipment.

Industry officials said the mandatory congressional notification process began before the recent flare-up in tensions with North Korea, but the timing of the Pentagon's announcement came a day after Pyongyang's surprise announcement that it would restart a long-shuttered nuclear reactor.

The F-35, F-15 and Eurofighter Typhoon are locked in competition to supply South Korea with 60 fighter planes.

Industry sources and US government officials expect Seoul to announce its decision between June and November.

Eurofighter is built by EADS, Finmeccanica SpA and BAE Systems.

The Pentagon agency said the Lockheed F-35 fighter sale would be worth up to US$10.8 billion (S$13.4 billion), including engines built by Pratt & Whitney, a unit of United Technologies Corp.

One industry source familiar with the sale said the total sum was "conservative" and the final selling price of the F-35 and associated equipment would likely be lower.

Lockheed welcomed the congressional notification about the possible sales, but underscored that Seoul was still continuing to evaluate all three proposals.

Spokeswoman Laura Siebert said the F-35 would give South Korea "all-aspect stealth with the most advanced avionics ever integrated into a fighter aircraft providing a quantum leap in capability over all 4th generation aircraft".

DSCA said the foreign military sale portion of Boeing's modified F-15 fighter would be worth up to US$2.4 billion.

That includes only the direct government-to-government sale of equipment for the jets, but not the actual planes - which would be sold to South Korea as part of a direct commercial sale.

One industry source said the total cost of the Boeing proposal was believed to be lower than that of Lockheed's.

Boeing declined comment on details of its proposal to South Korea, but said it was confident that is F-15 Silent Eagle offering was "best suited" to address Seoul's requirements.

South Korea already operates a fleet of Boeing F-15 fighters, but the company's new Silent Eagle model includes some stealthy characteristics.

Boeing spokesman Karen Fincutter said the newest F-15 model included additional capabilities that offered Seoul "an unprecedented blend of balanced survivability and lethality to meet customer needs in all phases of air combat".