Newt Gingrich, a top Donald Trump supporter and former speaker of the House, defended on Thursday the Republican nominee's recent remarks setting conditions for honoring the NATO military alliance.

"[NATO] countries ought to worry about our commitment," Gingrich said on "CBS This Morning" in an interview. "They ought to worry about commitment under any circumstances. Every president has been saying that the NATO countries do not pay their fair share."

Of countries that aren't "doing their fair share," Gingrich said, "They're a source of weakness, not a source of strength."

When pressed on whether Trump and the U.S. would come to the aid of countries like Estonia in the case of a Russian invasion, Gingrich said he too "would think about it a great deal."

"Estonia is in the suburbs of St. Petersburg. The Russians aren't gonna necessarily come across the border militarily. The Russians are gonna do what they did in Ukraine," he said. "I'm not sure I would risk a nuclear war over some place which is the suburbs of St. Petersburg. I think we have to think about what does this stuff mean."

When asked whether he believed Trump meant to terminate the NATO agreement, Gingrich said he believed the GOP nominee wanted to have a "very serious conversation about us being the people who defend people who won't defend themselves."

At the convention Wednesday night, Gingrich spoke shortly after Texas Sen. Ted Cruz, whose speech rebuffed the GOP nominee with a non-endorsement.

In his own address, Gingrich attempted to clarify to the audience that Cruz was actually pro-Trump, telling delegates upset over the Texas Republican's speech that "I think you misunderstood one paragraph" of his address.

He tried to spin it positively, saying Cruz had told them to "vote your conscience for anyone who will uphold the Constitution." But, Gingrich then said, "in this election there is only one candidate who will uphold the Constitution."

Of his off-the-cuff remarks that night, Gingrich said Thursday that "it was a little bit on the fly, but I was happy to do it."

Asked why he felt the need to do so, the ex-House speaker said, "If you watch the crowd out here, they were unhappy and restive."

He noted the delegates needed "closure," and he believed he had to do something to provide it.

Gingrich also defended Trump's decision to allow Cruz speaking time when the billionaire knew he wouldn't be getting an endorsement.

"I think Trump made the right decision," he said. "You either don't let him speak, which that then becomes a huge story... or you let him take the risk."