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 US 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."

Earlier on Thursday, NATO head Jens Stoltenberg told Ukrainian President Petro Poroshenko that NATO would deliver secure communication equipment to Ukraine's military this month.

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."

Foreign Ministry State Secretary Levente Magyar told the parliamentary committee that the differences between the US and Hungary were not strategic, and said he hoped dialogue would help rectify them, according to state news agency MTI.

Hungary's Prime Minister Viktor Orban has backed Western sanctions on Russia over its annexation of the Crimean peninsula and its backing for a pro-Russian separatist revolt in eastern Ukraine, but he has also criticised the measures.