BANGKOK, March 23 (Reuters) - Thailand must submit written comments to the United States on the U.S. Section 301 trade investigation by April 15, the commerce ministry said on Monday.

The country risks facing a tariff if it fails to defend its case, Chotima Iemsawasdikul, director general of the Department of Trade Negotiations, told a briefing.

* Thailand is among 16 countries the US said would be subject to investigation underSection 301 of the Trade Act of 1974. * The investigation of Thailand covers excess productioncapacity in sectors such as autos, machinery parts, fish, fishoil, animal feed and garments, and a separate case on imports ofgoods made with forced labour, with no specific source countryidentified, Chotima said. * The country remains in discussions with the US followingthe original joint statement on tariff and non-tariff issues, she said. * Thai goods had been hit with a 19% US tariff, which hasnow been replaced by Washington's new global tariff . * The Thailand-EU free-trade agreement talks are halfwaythrough, and Thailand is pushing to conclude them this year,Chotima said.