NATO members should not tolerate deliberate actions of hybrid war, and the United States will stand by countries as they deal with these incidents, Matthew Whitaker, the U.S. ambassador to NATO, said on Saturday.

Whitaker said last month’s attempted drone attack at Germany’s Leipzig/Halle Airport, which Berlin has blamed on Russia, was more worrying than other recent incidents NATO members have faced due to Russian drones targeting Ukraine that strayed off course.

“Then you have what I would call the more concerning hybrid actions like we saw in Leipzig, like we have seen in Poland,” Whitaker said speaking on the sidelines of a business conference in Italy.

NATO members must be prepared, vigilant and also deliver a strong message when they are challenged by these events, he said.

“If we can identify who’s doing it — which Germany did — we make sure we express strongly that is unacceptable and we will not tolerate it,” Whitaker said.

Critical infrastructure, including energy assets, could be targeted and should be monitored by national authorities, he added.

The response to these challenges is primarily national, but the United States supports countries when they are dealing with these acts, he said.

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(Reporting by Francesca Landini; Editing by Sharon Singleton)