BRUSSELS, Nov 8 (Reuters) - The European Union is negotiating with Merck & Co (MRK.N) and Pfizer (PFE.N) over possible contracts to supply their experimental COVID-19 drugs, an EU official told Reuters, adding that talks with Merck were more advanced. Both U.S. drugmakers have developed antiviral pills that have shown promising efficacy in trials of adults with COVID-19 who are at high risk of serious illness, with Pfizer"s at 89% and Merck"s around 50%. read moreContacts are ongoing with both," the EU official familiar with the discussions said on Monday, asking not to be named as the negotiations are confidential. Talks with Merck were more advanced because it has already begun submitting data to the European Union"s drugs regulator, the official added.Pfizer has yet to submit any data because preliminary results of its trials were published only last week, a month after Merck"s first results. A spokesperson for the European Commission, which coordinates talks with drugmakers on behalf of EU states, declined to comment on negotiations. Merck was not immediately available for comment. Pfizer declined to comment. EU officials discussed procuring COVID-19 drugs at a meeting last week, the EU source said, stressing the bloc of 27 member states wanted to move as fast as possible to buy new treatments, but wanted to have further guarantees on their safety. So far, the only approval is for Merck"s drug in Britain. Both drugs treatments are given for five days. Pfizer"s regimen is for three pills taken in the morning and three at night. Merck"s is four pills in the morning and four at night. Asked whether the higher efficacy shown by Pfizer"s treatment could affect talks with Merck, the EU official said they could not be compared for now and more data was needed. A rush to secure the initial, limited courses of both pills is under way. Britain has procured 480,000 courses of Merck"s pill and 250,000 courses of Pfizer"s. Meanwhile, U.S. President Joe Biden said last week that the United States had secured "millions" of courses of the Pfizer treatment, and has also bought 1.7 million courses of Merck"s. Australia and several Asian countries have also secured courses of the new drugs or are in the process of doing so. read more Indonesia"s drug regulator said on Monday that it was reviewing Merck"s pill, ahead of a possible purchase of up to 1 million courses of it next month read more . Thailand, which has been in talks with Merck to buy 200,000 treatments since October, is also in negotiations with Pfizer for its pill, a medical adviser told Reuters. Pfizer expects to produce more than 180,000 courses of its therapy by the end of 2021, whereas Merck plans output of 10 million courses. Both plan to ramp up production in 2022. Reporting by Francesco Guarascio; additional reporting by Chayut Setboonsarng in Bangkok and Stanley Widianto in Jakarta; Editing by Alexander Smith