"If for some of our partners the most important thing is to reach the deal and then to work through security guarantees, for us it's now a precondition for reaching the deal," she said on Thursday in an interview at the alliance's headquarters in Brussels.

Such guarantees should be "well-developed" -- not mere "assurances" -- expressed in a "strong" language close to that of NATO's Article 5, and included in a treaty with the United States and possibly some European countries, she added.

A cornerstone of NATO, Article 5 provides that if one member is attacked, the entire alliance comes to its defence.

Under a coalition of the willing co-chaired by France and Britain, Ukraine allies are discussing providing security guarantees to Kyiv to deter Russia from further aggression, if a peace deal is reached.

Many see US participation as key for these to be effective, but Washington's willingness to chip in remains unclear.

Citing the deployment of a "multinational reassurance force", Getmanchuk said: "What we have to clearly understand now (is) what could be the US backstop here".

Ukrainian President Volodymyr Zelensky said Thursday he had discussed security guarantees with top US officials, without providing further details.

This came amid a flurry of diplomacy around a plan to end the conflict in Ukraine originally proposed by US President Donald Trump last month.

Separately, the European Union is debating using frozen Russian assets for a mammoth loan to support Kyiv -- something Getmanchuk described as a "game-changer".

"Putin will realise that Ukraine will not collapse as he's hoping," she said.

EU leaders are due to discuss a plan to tap some 200 billion euros ($232 billion) of Russian central bank assets immobilised in the bloc as they meet in Brussels next week.