BRUSSELS — U.S. Defense Secretary Chuck Hagel said Wednesday the European allies must start bearing more of the burden of modernizing NATO or risk it becoming irrelevant.

In remarks to the opening of a two-day defense ministers meeting at NATO headquarters, Hagel said the current path of declining European defense investment is not sustainable.

"As European economies recover, leaders must make the case for renewed investment in military capability," he said. "The current path is not sustainable. Our alliance can endure only as long as we are willing to fight for it,and invest in it."

He added, "If the alliance is to remain effective, adaptable, and relevant, rebalancing NATO's burden-sharing and capabilities is mandatory — not elective."

Hagel's remarks echoed warnings from his predecessors going back to the days of the Cold War. What's different now is that NATO is emerging this year from its first-ever protracted land war, in Afghanistan, whose costs and hardships create questions for many Europeans about the future role of the alliance.

"America's contributions in NATO remain starkly disproportionate, so adjustments in the U.S. defense budget cannot become an excuse for further cuts in European defense spending," Hagel said, according to prepared remarks provided to reporters, who were not allowed to attend his closed-door conference.

Hagel was attending a dinner Wednesday at which the ministers were expected to begin discussing events in Afghanistan and the prospects for a future U.S. and NATO military role there. The ministers were to be briefed on Thursday by Marine Gen. Joseph Dunford, the top U.S. and allied commander in Kabul.