Romania wants an explanation from Russia over threatening remarks made by a senior Kremlin official.
Russian Deputy Prime Minister Dmitry Rogozin said he would return in a strategic bomber after Romania barred his plane from its airspace.
Rogozin issued the statement on May 10 on Twitter. His plane was returning to Russia from the Moldovan breakaway region of Transdniester.
"Upon U.S. request, Romania has closed its air space for my plane," he tweeted. "Ukraine doesn't allow me to pass through again. Next time I'll fly on board TU-160."
The supersonic Soviet-era TU-160 is Russia's largest strategic bomber.
Rogozin is one of the Russian officials on Western travel-ban lists over Moscow's aggressive actions in Ukraine, including the annexation of Crimea.
Romania's Foreign Ministry asked Moscow to clarify whether Rogozin's comments represented "the Russian Federation's official position towards Romania as an EU and NATO member."
The Foreign Ministry also said Rogozin's tweet was "a very grave statement under the current regional context."
It added that "the Russian Federation has broken Ukraine's territorial sovereignty...while pro-Russian separatists are violating public order in the neighboring state."
The statement said Romania was abiding by the sanctions imposed by the European Union against Russia.
Rogozin was in Transdniester to attend celebrations marking the defeat of Nazi Germany in 1945.
Transdniester, a thin strip of land between Moldova and western Ukraine, declared independence from then-Soviet Moldova in 1990.
Moldovan forces and Moscow-backed Transdniester fought a short war in 1992 over fears that newly independent Moldova would seek reunification with neighboring Romania.
The conflict ended with a cease-fire agreement after Russian troops in the region intervened on the side of the separatists.
Some 1,400 Russian troops remain in Transdniester.
The standoff in Ukraine, where government troops have been fighting rebels in the Russian-leaning east following Russia's annexation of Crimea, has sparked fears Transdniester could be Moscow's next target.
NATO's top commander in Europe, U.S. Air Force General Philip Breedlove, warned in March that Russian troops massed on Ukraine's eastern border were well positioned to invade Transdniester.
With reporting by Reuters and ITAR-TASS