Army spokesman Simon Korez said in a statement that the 800-strong 72nd brigade had been assessed as "not combat ready" after a series of NATO tests.

"We have been warning for years what's going on. We showed we were being underfunded and that our duties had increased and that such a situation was unbearable in the long term," Korez said, adding the military's equipment is "old and worn out".

Slovenian media had reported on the failure earlier in the week, noting the brigade had 18 months of training for the tests.

Korez said additional measures would be taken to improve the situation and that preparing for a new test would be the unit's top priority.

Slovenia was the first state from the former Yugoslavia to join NATO in 2004 but, after having been severely hit by the global financial crisis, reduced defence spending by more than a third between 2010 and 2017 to one percent of GDP.

In 2016 President Borut Pahor expressed concern that cuts had left the military with "insufficient" capability to intervene in crisis or conflict situations.

Slovenia's government has committed to increase the defence budget in the coming years, with the aim of reaching 1.14 percent of GDP by 2024, according to Defence Minister Andreja Katic.

That will still fall short of the target set by NATO allies in 2014 of raising defence spending to two percent of GDP over a decade.