The US government on Thursday said economic sanctions over Turkeys planned purchase of a Russian missile defense system remained a "very viable" option, despite Turkish President Recep Tayyip Erdogan threatening to retaliate if that occurred. US Assistant Secretary of State for Political-Military Affairs Clarke Cooper told reporters on a teleconference that the United States and other NATO allies remained in discussions with Turkey about resolving the dispute. "Seeking resolution is still within the realm of possible today, but imposition of sanctions remains a course of action and a very viable one at this point," Cooper said during a visit to Brussels. In Istanbul, Erdogan, speaking at a rare news conference for foreign media, repeated that the purchase of Russian S-400 systems was a done deal and said the United States should think carefully before imposing sanctions on a NATO member. "I do not see any possibility of these sanctions happening," Erdogan said. However, if they did, "we will have sanctions of our own." "This business is finished for us and for Russia," he said. "The training of personnel has been completed ... I believe that in the first 15 (days) of next month, the delivery will take place." Washington has repeatedly threatened to impose sanctions unless Ankara backs down from the S-400 purchase. Erdogan, scheduled to meet President Donald Trump during a Group of 20 meeting in Japan next week, said he hopes to dissuade the US from imposing sanctions. Turkey and the United States have been at loggerheads over the issue for months. Washington says the S-400 is incompatible with NATOs defense network and could compromise its F-35 fighter jets, an aircraft Turkey is helping build and planning to buy. Turkey faces expulsion from the F-35 program, which would cost Turkish Aerospace Industry (TAI) and other companies well-paying orders, but the sanctions could be extended to other Turkish companies that produce components for US firms. Even minor US sanctions could prompt another sharp sell-off in the Turkish lira. A 30 percent slide in the currency drove the Middle Easts largest economy into recession last year. It has fallen another 10 percent this year, and markets remain on edge.