A NATO member said Ukraine joining the alliance is 'irreversible' as fears mount that Trump would block membership

• Estonia's president said Ukraine joining NATO is "non-negotiable" and "irreversible."
• NATO must send a strong message that it will support Ukraine for as long as it takes, Alar Karis said.

Estonian President Alar Karis said Ukraine joining NATO isn't negotiable and that the Western military alliance must send a strong message that NATO will support Ukraine for as long as it takes.

"Ukraine's NATO membership is non-negotiable, and the process of becoming a member is irreversible," he said, according to Estonian news outlet ERR.

Karis' comments came after reports that former President Donald Trump, now the presumptive Republican presidential nominee, would consider stopping Ukraine's NATO progress as part of a peace deal with Russia.

Trump-aligned national security experts told Politico last week that, if elected, Trump is considering negotiating a deal with Russian President Vladimir Putin that would see Russia getting some territory and NATO committing to not expanding membership to Ukraine or Georgia.

Sources told Politico that Trump is unlikely to quit NATO outright, but members worry that, if elected, he could downplay the US role, block aid for Ukraine, or try to strike an unpopular peace deal with Russia.

Ukraine and the majority of its allies don't want a peace deal that gives Russia territory. They say it's not right, as Russia invaded a sovereign country, and that Russia can't be trusted in such deals.

Ukraine has taken steps toward joining NATO but has not yet been given a clear road map. Estonia has been a strong backer of its membership, saying that it would help protect it from Russia.

In his comments, Karis said that "if Ukraine's territorial integrity and sovereignty are not respected, it will lead to new and larger wars in the future. Russia and other aggressors will only be emboldened to attack again."

"NATO has successfully overcome previous hesitations regarding military aid to Ukraine," he added. "No limits should be set on further assistance."

Being a member of NATO would offer Ukraine strong protection due to the alliance's mutual protection policy, which means that if one member is attacked it can call on other members, like the US and UK, for help.

Many NATO states have called for Ukraine to be granted membership of the alliance.

But any single member, such as a Trump-led US, could block it, as new membership needs consensus support.

Trump has long been critical of NATO, and threatened to withdraw from it during his presidency.

Trump long complained that many NATO members did not spend the recommended 2% of their GDP on defense.

(Members' spending has soared since — but experts told Business Insider that Trump's protests were not the reason.)

The Associated Press reported this week that European leaders are trying to "Trump-proof" NATO and its Ukraine support, in case he is reelected.

Member states like Estonia are also preparing in case Russia attacks them next.

"Russia will be more hostile in the next decade than it was in the previous one," Karis said. "NATO must be prepared for this. NATO has good collective defense plans, which need to be backed by necessary forces and weapons systems."