A "reform path for Ukraine" will be drawn up but "I can't put a timetable on it," Sullivan told reporters at the NATO summit in Vilnius, also announcing that President Joe Biden will meet Ukraine's President Volodymyr Zelensky on Wednesday.

Sullivan ruled out any immediate entry for Western-backed Ukraine, given its ongoing war against Russian invasion, saying this would "bring NATO into a war with Russia."

Citing "a lot of good will" for Ukraine's ambition at the summit in Vilnius, Sullivan said "the question is what's the pathway?"

"I think we can come to a good understanding," he said.

Asked whether NATO membership would be impossible as long as Russian troops remain on any Ukrainian territory, Sullivan left the door open to compromise.

"Today, we're not going to define how the war (ends), we're not going to lay down the definition for that," Sullivan said, noting "there have been historical circumstances where you've seen different ways of looking at that question."

Separate to the issue of membership, NATO countries are discussing concrete interim security measures to offer Ukraine, beyond the current aid pouring in to help Kyiv's military push back the Russians.

Sullivan said this would be discussed during a meeting Wednesday between Biden and Zelensky.

US officials say they expect a core group of allies within NATO to agree during the summit on a set of longterm security assistance measures for Ukraine to provide funding and weapons in the interim period while applying to become an alliance member.

The details of those measures remain sketchy but US officials have compared the idea to the longterm spending program to fund Israel's military.

Biden and Zelensky will discuss "how the US alongside our... partners are prepared to make longterm commitments to help Ukraine defend itself now and to deter future aggression," Sullivan said.

Sullivan also hailed the decision Monday by Turkey to finally drop opposition to Sweden's entry into NATO.

The White House national security chief said that "President Biden's personal leadership" had been key to getting Turkey's President Recep Tayyip Erdogan to stop blocking the Swedish bid, which required unanimity from all alliance members.

Biden called Erdogan from Air Force One on his way to London, before coming to the NATO summit, and was set to hold face-to-face talks in Vilnius with him later Tuesday.

Asked if Turkey's support for Sweden now cleared the way for the long-delayed sale of US F-16 fighter jets to Turkey, Sullivan said Biden had always supported the sale, which has faced opposition in the US Congress.

A US official told AFP that the White House is now "actively engaging" with Congress to clear the way to the sale, which is a top demand from the Erdogan administration.