Joe Biden raised “significant corruption” in an interview with Time magazine this year, during which he ruled out supporting the “Nato-isation of Ukraine”.
Nato diplomats and officials have given Ukraine a list of reforms it will be expected to carry out before its membership ambitions can be realised, a US defence official said.
“That’s something Nato has been doing quietly under the radar that helps them get closer to membership,” the source added.
Mr Zelensky, who is attending the summit in Washington, is likely to be offered a promise of a “well-lit bridge” to Nato membership, a term preferred by the US.
Nato allies still disagree over whether they should upgrade the statement they made last year at their summit in Vilnius, Lithuania, to make their offer to Kyiv “irreversible”.
At this year’s meeting, allies are expected to offer further air defence systems to meet Ukraine’s growing needs, including US Patriots, after Washington agreed to prioritise deliveries of its flagship system to Kyiv.
Despite the lack of Nato promises, almost 20 countries have signed bilateral security agreements with Ukraine. They will come together to celebrate those new pacts in a joint statement at the Nato summit
The summit is likely to be the first major international event for Sir Keir Starmer if he is elected as prime minister.
Writing in The Telegraph on Wednesday, Lord Cameron urged Labour to commit to spending 2.5 per cent of GDP on defence.