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 invading Russian forces, according to three sources familiar with the matter, Reuters reports.

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 Biden administration has been asking allies who have been using Russian made equipment and systems including S-300s and S-400s to consider transferring them to Ukraine as it tries to fend off a Russian invasion that began on Feb. 24. read more

The idea, which analysts said was sure to be shot down by Turkey, was part of a wider discussion between Sherman and Turkish officials about how the United States and its allies can do more to support Ukraine and on how to improve bilateral ties.

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.

Washington has repeatedly asked Ankara to get rid of the Russian-built surface-to-air missile batteries since the first delivery arrived in July 2019. The United States has imposed sanctions on a Turkey's defence industry and removed NATO member Turkey from the F-35 fighter jet programme as a result.

Ankara has said it was forced to opt for the S-400s because allies did not provide weapons on satisfactory terms.