British Prime Minister Rishi Sunak moved quickly to head off the suggestion that London was annoyed by President Volodymyr Zelensky's fierce lobbying at the NATO summit for more Western support in his battle against the Russian invasion.

And Zelensky himself brushed off any suggestions of tension in the relationship.

But, against a background of Ukrainian frustration that the two-day NATO summit in Vilnius had not produced an invitation for Kyiv to join the alliance on a clear timeframe, Wallace's comments triggered embarrassing headlines and suggestions of discord.

"There is a slight word of caution which is, whether we like it or not, people want to see gratitude," Wallace told British media on the sidelines of the NATO summit in Vilnius.

"Sometimes you're persuading countries to give up their own stocks. And yes, the war is a noble war and yes, we see it as you doing a war not just for yourselves but also our freedoms.

"But sometimes you've got to persuade lawmakers on the Hill in America," Wallace said referring to the US Congress.

"You have got to persuade doubting politicians in other countries that it is worth it, it's worthwhile and they are getting something for it," he added.

Wallace also recalled that after receiving a list of weapons requests from Ukraine last year, he told officials in Kyiv "I am not Amazon".

Asked about Wallace's remarks, Sunak said Zelensky "has expressed his gratitude for what we have done on a number of occasions".

"Not least in his incredibly moving address that he made to parliament earlier this year and he has done so again to me, as he has done countless times when I have met him," he told reporters in Vilnius.

- 'Call him today' -

"So I know that he and his people are incredibly grateful for the support we have shown, the welcome that we have provided to many Ukrainian families, but also the leadership we have shown throughout this conflict," Sunak added.

Zelensky was also asked about Wallace's comments at his summit press conference, and responded in an exasperated tone.

"I believe that we were always grateful to the United Kingdom. We are always grateful to the prime minister -- or to prime ministers -- and to the minister of defence," he told reporters, thanking the British public for their support and government for close cooperation.

Then, turning to Defence Minister Oleksiy Reznikov, Zelensky asked him whether he had a problem in his relations with Wallace. Told not, he replied to laughter: "So why don't you extend words of gratitude to him? That's great. Then please, you have to call him today."

On Wednesday, the second day of the NATO summit, the leaders of G7 nations pledged to offer "enduring" military support to help Ukraine defeat invading Russian forces.

Zelensky welcomed the security guarantees but did not disguise the fact that he would have preferred NATO to have agreed a clear timetable for Ukraine to join the alliance.