The United States will take a more hardline approach within Nato and openly press allies to meet their commitments, US defense secretary Pete Hegseth said on Thursday as Nato chief Mark Rutte confirmed that cuts to US contributions to the alliance's crisis-response forces had taken effect immediately, with other members moving to fill the gaps.

"The question yesterday came up: Is this immediate or not? It is immediate," Rutte told reporters ahead of a meeting of Nato defence ministers in Brussels.

"However, why I'm a little bit reluctant to say this is because it is a planning tool. So what would happen in reality? If war would break out...all allies, including the US, will max out what they can do to make sure we can fight the war."

Meanwhile, US defense secretary Pete Hegseth said on Thursday at Nato headquarters, that Nato needs to return its roots as a "hardline" military alliance and the United States will be candid in public and private about countries that need to do more to meet their commitments.

"(There are) some that still need to do more, and we will be candid about that, both in private and in public. I think that's important, friends being honest with friends," Hegseth said as he arrived for a meeting of defense ministers.