Munich, Germany ? NATO Secretary-General Lord Robertson warned the European and Canadian allies Sunday that a failure to improve their ability to respond to military crises could push Washington toward unilateralism.

The military gap between the United States and its European allies, an issue during the Kosovo and Bosnian conflicts, has become even more critical in the U.S.-led war on terrorism.

Speaking to a conference of defense officials and experts from more than 40 countries, Robertson noted that despite an annual investment of $140 billion by European NATO allies, Europe is “militarily undersized” and needs Washington’s support for major operations.

“American critics of Europe’s military incapability are right,” Robertson said. “So, if we are to ensure that the United States moves neither toward unilateralism nor isolationism, all European countries must show a new willingness to develop effective crisis management capabilities.”

Despite growing concerns that Washington will act alone, Robertson said the NATO alliance remained as critical to the fight against terrorism as it did in confronting the Cold War threat.

“Afghanistan reinforces the fact that no modern military operation can be undertaken by a single country,” he said. “Even superpowers need allies and coalitions to provide bases, fuel, airspace and forces. And they need mechanisms and experience to integrate these forces in a single coherent military capability.”

But he appealed to the United States to also help Europe modernize by making more technology available for export.

That drew a rebuff from Sen. Jon Kyl, a Republican from Arizona, who chided Europeans for developing their own Airbus to compete with Boeing’s transport planes and underlined U.S. concerns that military technology shared with its allies finds its way to rogue states or “other users who would exploit the power.”

Sen. Joseph Lieberman, a Democrat, emphasized the multilateral contributions to the Afghan campaign and the importance of the alliance’s first-ever vote to invoke a collective defense after the Sept. 11 attacks. But he warned of the dangers of the military gap.

“The gap isn’t just lingering. It is widening. Allowing it to persist threatens your security, puts a disproportionate burden on us and creates an awkward imbalance in our alliance.”

In his remarks to the conference, Russian Defense Minister Sergei Ivanov stressed that the war on terrorism requires a coordinated international response, including better intelligence sharing to help prevent future attacks.

“It is only the joint effort that can yield real fruit which would enable us to establish an effective system of defense security,” he said.

Ivanov will meet with Robertson in Rome on Monday to discuss how NATO and Russia can work together to combat terrorism and other issues.