“We assess that separatists in eastern Ukraine have acquired heavy weapons and military equipment from Russia, including Russian tanks and multiple rocket launchers,” Marie Harf, deputy spokeswoman for the State Department, said in a statement.

“Ukraine’s interior minister said three tanks crossed the border from Russia yesterday,” Harf said. “Internet videos showed this same type of tank that departed southwest Russia moving through multiple cities in eastern Ukraine.”

Harf added that Internet video has shown rocket launchers, which the department believes to have originated from the same deployment site as the tanks in southwest Russia, traveling through the Ukrainian city of Luhansk.

Earlier Friday, Ukrainian government forces were said to have retaken the Ukrainian port city of Mariupol from pro-Russia separatists in heavy fighting, and to have regained control of a long stretch of the border with Russia.

These advances would mean significant victories for Ukraine’s pro-European leadership, which is fighting to hold the former Soviet republic of 45 million people together. An armed separatist rebellion began in eastern Ukraine in April. The United States and other Western powers have accused Russia of orchestrating it.

"At 10:34 a.m. the Ukrainian flag was raised over City Hall in Mariupol," Interior Minister Arsen Avakov wrote on Facebook, less than six hours after the attack began on the city of 500,000, Ukraine's biggest Azov Sea port.

A ministry aide said the government forces stormed the rebels after they were surrounded and given 10 minutes to surrender. At least five separatists and two servicemen were killed in the battle before many of the rebels fled.

Mariupol, which has changed hands several times in weeks of conflict, is strategically important because it lies on a major road linking the rest of Ukraine to the southeastern border with Russia, and steel is exported through the port.

Avakov said government forces had won back control of a 75-mile stretch of the border that had fallen to the rebels, but it is not clear who controls other parts of the nearly 1,000-mile border with Russia.

The rebels, who have taken over several towns and cities and want eastern Ukraine to become part of Russia, confirmed five of their fighters were killed in the fighting for Mariupol.

Avakov said national guard, Interior Ministry and special forces units were involved in the battle.

A Ukrainian defense analyst, Dmytro Tymchuk, said four Ukrainian soldiers had been killed and 31 wounded in fighting in other parts of eastern Ukraine in the past 24 hours. The total death toll is unclear, but several hundred people have been reported killed in clashes this year in Kiev and in the east.