Former British Prime Minister Boris Johnson has urged the United States not to grow weary of Ukraine's grinding battle against Russia, telling CNN that victory over Moscow is imperative.

In an interview with CNN Tonight's Laura Coates as a key NATO summit concluded Wednesday, Johnson called on the US to continue its support of Ukraine, including the steady flow of weapons. He said that there could be "no possible excuse" to delay Ukraine's NATO membership.

"There can be no possible excuse or reason to keep faffing around and delaying," Johnson said, adding that it was "very important" to establish that Ukraine was on the path to NATO membership. "The last remaining objection was that it was going to be provocative to Vladimir Putin. Well, we've seen what happens when you don't have Ukraine in NATO, you provoke the worst war in Europe in 80 years."

US President Joe Biden and G7 leaders unveiled a substantial show of support for Ukraine Wednesday at the NATO Summit in Vilnius, Lithuania, offering a joint declaration of support aimed at bolstering Kyiv's military capability.

Biden acknowledged that the alliance did not invite Ukraine to membership during the summit as it works on "necessary reforms," but said it would continue to boost the country's security. Biden has emphasized that Ukraine is not ready to enter NATO, telling CNN in an exclusive interview last week that Russia's war in Ukraine needs to end before the alliance can consider adding Kyiv to its ranks.

When Zelensky arrived in Lithuania on Tuesday he issued a blistering statement expressing his frustration at not receiving more specific details on when and how Ukraine would join the alliance. Biden said on Wednesday that he had talked to Zelensky about the "kind of guarantees we could make in the meantime."

During his time as Britain's Prime Minister, Johnson was a vocal supporter of Ukraine and developed a close working relationship with Zelensky, becoming one of the first foreign leaders to make the precarious trip to Kyiv. Johnson resigned as Prime Minister in September 2022 and as a Member of Parliament in June, in the wake of scandals over his handling of the UK's coronavirus crisis.

On Wednesday he warned that aside from the Russian army, the biggest enemy that the Ukrainians have in the conflict is "Ukraine fatigue."

"It's the exhaustion of the rest of the world, and particularly the supporters of Ukraine, in making sure that they win."

"But they've got to win, it's absolutely crucial," he added, explaining that a Ukrainian victory is "vital for democracy and freedom around the world."

In the interview, Johnson also said that he was supportive of Biden's decision to provide Ukraine with anti-cluster munitions, despite the UK condemning the move. "I think President Biden has done the right thing to supply them and the faster the Ukrainians can use them to recapture their territory, the more lives will be saved."

However, he stopped short of calling on the UK government to also send the weaponry, pointing out that Britain is a member of the Convention on Cluster Munitions treaty. More than 100 countries, including the UK, France, and Germany, have outlawed the munitions under the treaty. However, the US and Ukraine are not signatories to the ban.

The White House announced on Friday that President Biden had approved the transfer of cluster munitions to Ukraine -- a controversial decision which was met with both praise and criticism.

When asked if the possible re-election of Donald Trump in the next presidential elections could jeopardize US support for Ukraine, Johnson pointed out that it was Trump who first approved sending Javelin anti-tank weapons to Ukraine in 2019, prior to the outbreak of war.

"Trump has a strong record already in helping the Ukrainians," he claimed.

Johnson did not mention, however, the circumstances surrounding Trump's hold up of military aid to Ukraine in 2019 that was at the heart of the first impeachment inquiry. In a phone call with the newly elected Zelensky in July 2019, Trump appeared to pressure him to investigate his then-Democratic rival Joe Biden in exchange for releasing the aid.