NATO's top general is siding with Sen. John McCain in a spat over whether the U.S. should vote to allow the tiny country of Montenegro to joint the alliance.

U.S. Army Gen. Curtis Scaparrotti, who is NATO's supreme military commander, testified Thursday that it was "absolutely crucial" that Montenegro be granted NATO membership.

"If we were to lose this, it would set back many of the other countries, and people, particularly in eastern Europe that are looking forward to, and have their eyes set on the West," Scaparrotti said

McCain is in a public and acrimonious fight with Kentucky Sen. Rand Paul, who objects to Montenegro's bid to join NATO, and is blocking an expedited Senate vote that would add the U.S. to the 26 NATO nations supporting Montenegro's bid to join the 28-nation alliance.

"There has to be the thought whether or not it's in our national interest to pledge to get involved with a war if Montenegro has an altercation with anyone," Paul said on MSNBC last week. "Whenever there's a war fought, our soldiers fight it and our dollars pay for it. And so the 45 soldiers that Montenegro has I think are hardly an asset to our national security," Paul said.

"Our decisions need to be about our national security, and so I just don't think it enhances our national security to have Montenegro part of NATO."

Paul's legislative maneuver, which would force a time-consuming roll call vote, infuriated McCain last week, who then berated Paul on the Senate floor.

"That is really remarkable that a senator, blocking a treaty that is supported by the overwhelming number, perhaps 98, at least, of his colleagues, would come to the floor and object and walk away," McCain said, then adding a final zinger, "The senator from Kentucky is now working for Vladimir Putin,"

Paul responded to McCain's vitriol in the MSNBC interview the next day. "I think he makes a really, really strong case for term limits," Paul said. "I think maybe he's past his prime; I think maybe he's gotten a little bit unhinged."

Scaparrotti testified Thursday that Putin, who opposes Montenegro's membership, has already told his inner circle that while he may have lost Montenegro, "there won't be another one."

Amid questions about whether the Trump administration would stand up to Moscow, Secretary of State Rex Tillerson sent a letter to Senate leaders this month saying that admitting Montenegro is "strongly in the interests of the United States."

Tillerson called for the Senate to act before the NATO summit in May so that Montenegro may take a seat as a full member.