The United States has informally discussed with Turkey the unlikely possibility of sending its Russian-made S-400 anti-missile systems to Ukraine to help it fight invading Russian forces, Reuters reported, citing its own sources.

U.S. officials have discussed the proposal with their Turkish counterparts over the past month, but no specific or formal requests have been made, the sources told Reuters. They said the issue was also discussed briefly during U.S. Undersecretary of State Wendy Sherman's visit to Turkey in early March.

The Joe Biden administration is asking its allies that use Russian-made equipment and systems, including the S-300 and S-400, to consider transferring them to Ukraine.

The idea, which analysts say is certain to be rejected by Turkey, was part of a broader discussion between Sherman and Turkish officials about what more the U.S. and its allies could do to support Ukraine and how to improve bilateral ties.

Turkish authorities have not commented on any U.S. offer to transfer Ankara's S-400 systems to Ukraine, which have been a subject of long-standing disagreement between the two NATO allies.

Turkish sources and analysts have said that any such proposal would not satisfy Turkey, citing various technical difficulties associated with installing and operating the S-400s in Ukraine and political problems such as the blow Ankara would likely receive from Moscow.

U.S. officials are keen to seize this moment to draw Turkey back into Washington's orbit. Efforts to find "creative" ways to improve the strained relationship have accelerated in recent weeks, even though no specific proposal has so far gained traction, U.S. and Turkish sources have said.

"I think everyone knows that the S-400 has been a long standing issue and perhaps this is a moment when we can figure out a new way to solve this problem," Sherman told Turkish broadcaster Haberturk in an interview on March 5.

It was unclear exactly what she meant, and the State Department did not respond to questions about her comments.

The White House did not respond to a request for comment about the suggestion made during her visit to Turkey.

The effort is also part of a wider bid by the Biden administration to respond to Ukrainian President Volodymyr Zelenskiy's plea to help protect Ukraine's skies. Russian or Soviet-made air defense systems such as S-300s that other NATO allies have and S-400s are sought after.

One source familiar with U.S. thinking said Washington's floating of the possibility came as a result of the renewed effort to improve ties at a time when Ankara has been spooked by Russia's invasion of Ukraine.

Turkish President Erdogan had not received a specific heads up from Russian President Vladimir Putin on his plans of a full-scale attack on Ukraine, another source familiar with the discussions said.

Turkey shares a maritime border with Ukraine and Russia in the Black Sea and has good ties with both. It has said the invasion is unacceptable and voiced support for Ukraine, but has also opposed sanctions on Moscow while offering to mediate.

Ankara has carefully formulated its rhetoric not to offend Moscow, analysts say, with which it has close energy, defence and tourism ties. But Ankara has also sold military drones to Kyiv and signed a deal to co-produce more, angering the Kremlin. Turkey also opposes Russian policies in Syria and Libya, as well as its 2014 annexation of Crimea.

"Turkey has managed to walk on the razor's edge and a transfer of a Russian S-400 would certainly lead to severe Russian ire," said Aaron Stein, director of research at the Philadelphia-based Foreign Policy Research Institute. "And for Erdogan, the S-400 has become a symbol of Turkish sovereignty, so trading it away wouldn't be all roses and flowers."