WASHINGTON, June 11 (Reuters) - The largest U.S. titanium producer on Tuesday asked that the light-weight metal used in military aircraft and other defense equipment be excluded from free trade talks with Japan, arguing a healthy domestic industry was vital for national security.

"Titanium is a strategically important metal," Harry Seiner, a vice president at Titanium Metals Corp (TIMET), told the U.S. International Trade Commission. It was holding a hearing on the economic impact of Japan joining talks with the United States and ten other countries on a proposed regional free trade agreement known as the Trans-Pacific Partnership (TPP) pact.

Eliminating the 15 percent U.S. tariff on titanium would let Japan flood the U.S. market with titanium sponge, a semi-processed form of the metal that is used to make titanium ingots and slabs, Seiner said.

That would discourage further U.S. investment in the sector and cause many domestic sponge plants to close, he said.

Finished titanium is used in many of the most demanding military applications such as jet engines, naval vessels, satellites, vehicle armor, stealth aircraft and nuclear power.

But TIMET's request is at odds with the current goal among TPP members of negotiating a comprehensive agreement that phases out tariffs on all products.

Japan is one of the few major producers of titanium along with the United States, China, Russia and Kazakhstan.

Its industry is "very powerful" and has already made significant inroads into the U.S. market, even with the 15 percent duty, Seiner said.

U.S. imports of titanium sponge from Japan reached $241 million in 2012, compared to $33 million in 2002, he told the panel.

Negotiations on the proposed TPP agreement are in the fourth year, but Japan won't formally join the talks until the 18th round is held in July.

U.S. trade officials are still consulting with Congress and interested groups on negotiating objectives for Japan.

The United States has a history of treating titanium differently than other metals in some trade programs because of its strategic importance.

However, Washington agreed in a free trade pact with South Korea to phase out the tariff over three years.

A similar provision in the TPP agreement would devastate the U.S. industry, Seiner said.