Amid the controversy over Donald Trump's demand that NATO nations meet their financial obligations before they expect the United States to defend them, NATO's top commander says he agrees it's important for alliance members to pay their bills.

In an interview at the Aspen Security Forum, Army Gen. Curtis Scaparrotti said it's something he regularly discusses with other countries.

"What I tell them typically is that look, we have a people and a Congress with a tough budget as well," Scaparrotti said Thursday night. "So as a leader of another country you have to put your share into this, you have to do that because when I go back to testify to Congress I get asked these questions."

Pressuring NATO members to meet their commitments is a position Trump has not backed off of, despite criticism from Democrats and some national security experts, who argue it sends the wrong message and could embolden Russian President Vladimir Putin, who has already seized territory in Ukraine, which is not a NATO member.

Speaking in Miami Wednesday, Trump said, "I think NATO's great. But it's got to be modernized, and countries that we're protecting have to pay what they're supposed to be paying."

Scaparrotti, NATO's supreme commander, says as of now only five of the 28 NATO nations are meeting their pledge to spend at least 2 percent of their gross domestic product on defense.

They also are supposed to spend 20 percent of that on major weapons systems and equipment.

Only three countries have met that goal: the U.S., the United Kingdom and Poland. Two more, Greece and Estonia, spend 2 percent on defense, but not 20 percent on major systems.

Without mentioning his name, Scaparrotti agreed with Trump's point that without all countries in the alliance paying their fair share, it can't be militarily effective.

"It's very important," he said of the need for members to provide adequate funding for the alliance. "Without it we're not going to outpace Russia's modernization. We've got to have that in order to provide a credible force."

But Scaparrotti stopped short of saying the commitment to defend any NATO nation should be used as leverage to induce payments.

He called says the U.S. commitment to Article 5 of the NATO charter, which specifies an attack on one member is an attack on all, must be "ironclad" and that member countries should not be in doubt about that.

"One thing they need to know is that they can count on the United States to do what we say we're going to do," Scaparrotti said.