Videos posted on social media sites Monday show a series of rockets being launched next to an apartment complex in the war-torn city of Donetsk, Ukraine.
There are conflicting reports, though, as to whether pro-Russia separatists or Ukraine's security forces were behind the action.
While a cease-fire agreement remains in effect for eastern Ukraine, Russia's actions are worrying global leaders.
On Monday, NATO Secretary-General Jens Stoltenberg accused Russia, which annexed Ukraine's Crimea Region in March after an uprising against the Ukraine government, of violating the agreement. More than 4,300 civilains and soldiers have died in violence in Ukraine since mid-April.
"We see a significant military buildup in and around Ukraine," Stoltenberg said. "Large transfers of Russian advanced weapons, equipment and military personnel to violent separatists."
Kiev said Sunday that a convoy of 106 vehicles entered eastern parts of the country from Russia without its permission. Ukrainian officials also accused Russia of using humanitarian aid shipments to send weapons to rebels, Reuters reports. Russian officials deny those claims.
Ukrainian Foreign Minister Pavlo Klimki, who recently urged the West to begin sending weapons to the beleagured country, fought back against claims by Russia that American and European military aid would escalate tensions.
"It is a mistake by our European and American partners to say providing weapons and military technical assistance would create the potential for escalation," Klimkin told Reuters. "On the contrary, it would be an important step towards de-escalating the situation because no-one would go further if Ukrainian military forces get stronger."
While the U.S. has imposed tough sanctions against Moscow, its stopped short of aiding Ukraine with arms.