Russia sent armored tanks across the Ukrainian border on Friday, according to the U.S. State Department, after it recently accused Ukraine of doing the same.
Separatists in Ukraine were supplied heavy weapons, tanks and rocket launchers from Russia, Marie Harf, deputy spokeswoman for the State Department said.
“Russia will claim these tanks were taken from Ukrainian forces, but no Ukrainian tank units have been operating in that area. We are confident that these tanks came from Russia,” Harf said.
Online video showed the tanks departing from southwest Russia and moving through cities in eastern Ukraine, including Snizhne, Torez, and Makiyivka, Harf said.
Earlier, Russia accused Ukraine of sending two armored trucks across their border into the Rostov region.
“Such provocations” were a deterrent to the effort to establish a dialogue between Ukraine and Russia, the Russian Foreign Ministry said in a statement.
Moscow sent a “note of protest to Ukraine,” said the Russian Foreign Ministry, which called Ukraine's actions a “gross violation of fundamental rules of international law” and “an illegal act which will not contribute to a peaceful settlement of the armed conflict in the South-East of Ukraine.”
Russian news agencies reported that the Ukrainian troops fled Rostov, but left one of the vehicles behind because it had broken down.
Directly across the border, Ukrainian government forces reclaimed the port city of Mariupol from pro-Russian separatists after hours of fighting.
"At 10:34 a.m. (3:34 a.m. ET), the Ukrainian flag was raised over City Hall in Mariupol," Interior Minister Arsen Avakov wrote on Facebook. At least five separatists and two servicemen were killed before many of the rebels fled, according to Reuters.
Regaining control of Mariupol is important to the Ukrainian government because it serves as a portal for steel, and Ukraine says Moscow of uses the city as a gateway for armored vehicles.
The newly elected President Petro Poroshenko vowed to intensify defense against the pro-Russian separatists following months of unrest in the region and Russia's annexation of Crimea in March.