By Lili Bayer and Andrew Gray

BRUSSELS, Oct 16 (Reuters) - Ukraine is asking for a NATO membership invitation before Joe Biden leaves the White House, arguing it would be a fitting legacy for the U.S. president, a senior Ukrainian diplomat said.

"Our idea is that giving Ukraine (an) invitation at this moment is a political signal," Nataliia Galibarenko, Kyiv's ambassador to NATO, told Reuters.

"We sincerely believe that it can be part of the legacy of (the) current American administration," she said in an interview at the Ukrainian mission to NATO, opposite the alliance’s headquarters in Brussels.

Next month's U.S. presidential election creates major uncertainty for Ukraine as Washington has been Kyiv's biggest provider of military aid for its war against Russia's invasion.

While Democratic Vice President Kamala Harris has signalled continuing support for Ukraine, she has not spelled out the level of that support. Republican ex-President Donald Trump has not been clear about how he would handle the war.

Being in NATO would make Ukraine part of the U.S.-led military alliance's mutual defence pact, which means an attack on one member is considered an attack on all, although the accession process can take years of adapting to NATO standards.

Ukrainian President Volodymyr Zelenskiy has made clear in recent weeks that he sees an early NATO invitation as part of his "victory plan".

In the interview, Galibarenko elaborated on the rationale for Kyiv's push. She argued an invitation would remove a major bone of contention between Kyiv and Moscow.

Moscow has cited Ukraine's potential membership of NATO to justify the war while Ukraine has insisted it needs to join the alliance to protect against any future Russian aggression.

"If we will say that the invitation is there, for the Russian Federation, it would be like a final verdict – so that's it, so you cannot raise the stakes using this topic anymore," Galibarenko said in the interview, which took place on Tuesday.

NATO has declared Ukraine will join its ranks and its path to the alliance is irreversible. But it has said Kyiv cannot join while at war and declined to put a timeline on membership.

Most NATO members, including its predominant power, the United States, have shown no willingness to extend a membership invitation to Ukraine at this stage.

A senior U.S. government official, briefing reporters on condition of anonymity, said last week that there has been no shift in Washington’s position.

Galibarenko said Kyiv is not pushing for an immediate launch of membership talks but a formal invitation now would send a powerful message.

Asked when Ukraine would like the invitation to be issued, she replied: "The sooner the better."

(Reporting by Andrew Gray and Lili Bayer; editing by Philippa Fletcher)

((Lili.Bayer@thomsonreuters.com))