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The United States has informally raised with Turkey the unlikely possibility of sending its Russian-made S-400 missile defense systems to Ukraine to help it fight Russian forces, Reuters reported, citing three sources familiar with the matter.

U.S. officials have floated the suggestion over the past month with their Turkish counterparts but no specific or formal request was made, the sources told Reuters. They said it also came up briefly during Deputy Secretary of State Wendy Sherman's visit to Turkey earlier this month.

The Turkish authorities have not commented on any U.S. suggestion or proposal relating to the transfer to Ukraine of Ankara's S-400 systems, which have been a point of long-standing contention between the two NATO allies.

Turkish foreign ministry officials were not immediately available for comment.

Turkish sources and analysts said any such suggestion would be a non-starter for Turkey, citing issues ranging from technical hurdles related to installing and operating the S-400s in Ukraine, to political concerns such as the blowback Ankara would likely face from Moscow.