By Siyi Liu, Arathy Somasekhar

SINGAPORE/HOUSTON (Reuters) -U.S. exporters face potential disruption to their shipments of petrochemical feedstock ethane after the Commerce Department told them to seek licenses to export to top buyer China, according to trade sources and shipping data.

Chinese petrochemical producers rely on the United States for almost all their ethane imports, buying about half of the total U.S. exports of the gas.

Washington ordered a broad swath of companies to stop shipping goods to China without a license and revoked licenses already granted to some suppliers, Reuters reported on Wednesday. Those goods included ethane, as well as butane gas.

If U.S. exporters are unable to obtain licenses quickly, they will need to seek alternative buyers. Costs for Chinese petrochemical makers will rise as they compete for alternative sources of ethane or switch to another more expensive petrochemical feedstock such as naphtha.

China imported a record 230,000 barrels per day (bpd) of ethane from the United States last year, according to the U.S. Energy Information Administration. Ethane is a byproduct of oil and gas production and is primarily used to make plastics.

The exports have been caught in the trade war between the U.S. and China. Last month, China increased levies on imports of U.S. goods to 125% but waived the tariff for petrochemical producers.

At least two Very Large Gas Carriers (VLGC) were waiting at U.S. ports to load ethane this week while 15 more tankers are headed to, or waiting off, the U.S. Gulf Coast, to load about 284,000 bpd of ethane in June, Kpler data showed.

"It's going to be a major issue if all exports are suspended," said a Chinese ethane importer, who sought anonymity because he is not authorized to speak to media.

"We are cautiously watching if exporters can obtain new export licenses soon."

Petrochemical producer Ineos was scheduled to load ethane on VLGC Pacific Ineos Grenadier from Enterprise Products Partners' terminal in Morgan's Point, Texas, on May 24 for export to China, according to Kpler shipping tracking data. The ship docked on May 24 but has yet to load, according to LSEG tracking data.

Enterprise, a top exporter of ethane, said in a regulatory filing on Thursday that it had received a letter from the Commerce Department on May 23 requiring a license to export ethane and butane to China.

Enterprise said it was evaluating its procedures and internal controls and could not determine if it would be able to obtain a license.

Enterprise and Ineos did not respond to requests for comment. The U.S. Commerce Department did not immediately respond to a request for comment.