The U.S. ambassador to NATO, Kay Bailey Hutchison, said Tuesday that "we do not want this (cooperation) to be a protectionist vehicle for EU."
She said Washington is "going to watch carefully, because if that becomes the case then it could splinter the strong security alliance that we have."
EU leaders - 22 of whose nations are also members of the U.S.-led NATO alliance - agreed last year to jointly develop or purchase military equipment like drones. Washington is concerned now that the bidding process might exclude U.S. firms.
EU countries also drew up a list of criteria and binding commitments to set their cooperation in stone, rather than rely on the vaguer promises of the past. On top of that, they agreed to use EU funds to finance Europe's battle-groups - small, expeditionary forces that can rapidly be deployed to crisis hotspots.
Hutchison called for a transparent contract bidding process.
"We want the Europeans to have capabilities and strength, but not to fence off American products of course, or Norwegian products, or potentially U.K. products (once Britain leaves the bloc)," she said.
NATO and the EU have been trumpeting their cooperation on things like crisis management, the development of military equipment, maritime security and coping with hybrid warfare and cyberthreats.
They have constantly underlined that their aim is to complement, rather than compete with, each other.
"More European defense spending and capabilities can strengthen NATO and contribute to fairer burden-sharing, but only if the EU's efforts are developed as a complement and not an alternative to NATO," alliance Secretary-General Jens Stoltenberg said Tuesday, on the eve of a meeting between U.S. Defense Secretary Jim Mattis and his European and Canadian counterparts in Brussels.
A senior Pentagon official also said that Washington is concerned that EU defense cooperation might eventually draw resources away from NATO, which the U.S. and allies like Britain see as Europe's top security apparatus.
"Thus far we don't see signs that that is actually going to be a concern," said the official, Katie Wheelbarger. "But we just want to make sure that there has to be full transparency, so it's implemented right, so, therefore, future initiatives will be based on a positive example."
Lolita Baldor in Washington contributed.
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