Blasting Russia’s “dangerous escalation” in the region, U.S. planes on Thursday conducted an “extraordinary flight” over Ukraine in what Pentagon officials cast as a blunt warning to Moscow.

The flights come as a direct response to last week’s incident in the Kerch Strait, which saw Russian forces fire on Ukrainian vessels and take their crews hostage. The events sparked an international uproar and led President Trump to cancel a planned meeting with Russian President Vladimir Putin at last week’s G20 Summit.

In a statement on Thursday, the Defense Department said the flights were designed to send a clear message.

“Today, the United States and allies conducted an extraordinary flight under the Open Skies Treaty. The timing of this flight is intended to reaffirm U.S. commitment to Ukraine and other partner nations,” the Pentagon said in a statement. “The United States is resolute in our support for the security of European nations.”

“Russia’s unprovoked attack on Ukrainian naval vessels in the Black Sea near the Kerch Strait is a dangerous escalation in a pattern of increasingly provocative and threatening activity,” the statement continued. “The United States seeks a better relationship with Russia, but this cannot happen while its unlawful and destabilizing actions continue in Ukraine and elsewhere.”

The White House has come under increasing pressure to respond forcefully to Russia’s aggression in the region, which began with the 2014 forced annexation of Crimea and continued last week with the incident in the Kerch Strait.

The Pentagon also reportedly may send warships to the Black Sea.