Washington DC - Donald Trump's last remaining Republican rival for the election in November bashed the ex-president Sunday for his continued silence over the death of Russian opposition leader Alexei Navalny and his recent outburst over NATO.

"The fact that he won't acknowledge anything with Navalny – either he sides with [Russian President Vladimir] Putin and thinks it's cool that Putin killed one of his political opponents, or he just doesn't think it's that big of a deal," Nikki Haley said on ABC's This Week.

"Either one of those is concerning. Either one of those is a problem," added the Republican candidate, who is trailing far behind Trump in the race for their party's nomination.

Navalny's still-unexplained death at 47 in a prison in Russia's Arctic has drawn powerful condemnations from leaders around the world, starting with President Joe Biden, who has squarely blamed Putin.

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But Trump, Biden's likely opponent in November, has yet to say a word about it at any of several public appearances since Navalny's death was reported Friday.

The Trump campaign, asked for comment, has directed reporters to a post on Trump's Truth Social platform that says, "America is no longer respected because we have an incompetent president who is weak and doesn't understand what the World is thinking."

The post does not mention Navalny, Russia, or Putin.

The lack of comment comes days after Trump stunned Western allies by saying he would "encourage" Russia to attack members of the NATO military alliance who had not met their financial obligations.

The suggestion cast a pall over a major global security conference in Munich, drawing a warning from NATO Secretary-General Jens Stoltenberg that Trump should not "undermine" the alliance's security.

Biden also lashed out at Trump's remark as "dangerous" and "un-American."

Haley, the former UN ambassador under Trump, has not spared Biden from foreign policy criticism, but she called her former boss' NATO comment "bone-chilling."

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"All he did in that one moment was empower Putin," she told ABC.

"He sided with a guy that kills his political opponents, he sided with a thug that arrests American journalists and holds them hostage, and he sided with a guy who wanted to make a point to the Russian people: 'Don't challenge me in the next election or this will happen to you.'"

She said the episode underscored the need for Congress to pass a bill to provide military assistance to Ukraine after Trump successfully urged Republicans to oppose an earlier bill that included aid for Kyiv.

"Let's remind the American people that Putin said once he takes Ukraine, Poland and the Baltics are next... Those are NATO countries. That immediately puts America at war," she said.