QUITO, April 16 (Reuters) - Ecuador's state oil company Petroecuador has asked three of its Asian clients to demonstrate that they were not involved in a bribery scheme linked to commodities trading house Gunvor Group, according to documents seen by Reuters on Friday.

The requests come after a former Gunvor employee early this month pleaded guilty to U.S. charges of bribing Ecuadorean officials to win business with the company. Petroecuador nixed Gunvor from its list of authorized suppliers in response.

In the letters dated April 7, Petroecuador asked China's Unipec Asia and Petrochina International, as well as Thailand's PTT International Trading, to produce affidavits confirming that the companies and their employees "did not have anything to do with the possible acts of corruption" involving Geneva-based Gunvor.

"Until the company sends the requested information, they will not be invited to participate in the international tenders for the purchase and sale of hydrocarbons conducted by this public company's international trade division," all three letters read.

Unipec, Petrochina and PTT are among Petroecuador's largest customers, having signed long-term crude supply contracts tied to loans granted to former President Rafael Correa's government.

In the letters, Petroecuador said that the Gunvor's alleged bribe payments to Ecuadorean officials were made "in exchange for contracts signed by Petroecuador with Asian companies," without providing further details.

A Petroecuador spokesman said the company had received a response from Petrochina. In an April 13 letter seen by Reuters, Petrochina told Petroecuador it had no involvement with the alleged bribe payments and no corporate relationship with Gunvor.

Neither Petrochina's parent company Petrochina Co ltd , Unipec's parent company Sinopec or PTT's parent company PTT PCL immediately responded to requests for comment from Reuters sent after business hours in China and Thailand on Friday. (Reporting by Alexandra Valencia in Quito Writing by Luc Cohen Editing by Marguerita Choy)