An independent watchdog on Thursday told AFP it had seen weapons being transported from Russia to rebel-held areas of eastern Ukraine over the last week, contradicting Moscow's claim it is not arming separatists.
The Organisation for Security and Cooperation in Europe said it had observed convoys of cargo trucks moving across the border under the cover of night.
A drone captured footage of "vehicles, including a truck carrying an armoured personnel carrier, entering and exiting Ukraine" using an unpaved route with no border crossing facilities, it said.
The OSCE has previously complained of its drones being jammed.
On two other occasions it had seen weapons convoys moving near, but not crossing, the border.
In response to the new reports, the United States urged the Kremlin to "stop providing deadly weapons under cover of night to its proxies in eastern Ukraine."
"Only Russia can bring an end to bloodshed in Ukraine," the US embassy in Ukraine said last week, after the OSCE announced it observed the first of the convoys.
More than 10,000 people have been killed since the conflict with Moscow-backed rebels broke out in April 2014 following Russia's annexation of Crimea from Ukraine.
Ukraine and its Western allies accuse Russia of funnelling troops and arms across the border.
Moscow has denied the allegations despite overwhelming evidence that it has been involved in the fighting.
The OSCE team's 600 members are the only independent monitoring mission in the war-torn area who provide daily reports on the fighting.