US Defense Secretary Chuck Hagel is starkly warning European allies that they must bear more of the burden of modernizing NATO or risk it becoming irrelevant.

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

He said NATO can function only as long as its member countries are willing to fight for it and invest in it.

Hagel said planned reductions in the size of the U.S. Army and other proposed defense savings outlined in Washington this week should not be seen by Europeans as an excuse to reduce their military capabilities.