The United States urged Hungary on Thursday to repair relations with neighboring Ukraine and not try to block Kiev’s cooperation with NATO, saying “if Ukraine fails, Hungary will be on the front line of Russian aggression”, Reuters reported.

Hungary accuses Ukraine of trampling over the educational rights of its ethnic Hungarian minority. Budapest has threatened to retaliate by blocking Kiev’s efforts to move closer to NATO and the European Union, clubs Hungary belongs to.

The U.S. ambassador in Budapest, David Cornstein, told a parliamentary committee that Hungary could do more to “prioritize alliances” at a time when he said Russia posed a renewed threat to Ukraine and to Europe.

“Now more than ever, Russia is testing the West,” Cornstein said in a statement circulated by the embassy.

“We feel strongly ... that as NATO allies the best way to promote reforms in Ukraine is by talking to Ukraine, not by blocking Ukrainian engagement with NATO.”

Cornstein said Hungary, which is heavily reliant on Russian oil, gas and nuclear expertise, was playing with fire by cosying up to President Vladimir Putin.

“We need to keep the big picture in mind,” he said. “Putin is not interested in national sovereignty. His vision is neoimperial. If Ukraine fails, Hungary will be on the front line of Russian aggression.”