About 300 US Army paratroopers have arrived in Ukraine for training exercises with national guard units - a move that Moscow and eastern Ukraine's pro-Russian separatist rebels have warned will escalate the conflict.
The troops, from the Italy-based 173rd Airborne Brigade, are to spend six months training a total of about 900 Ukrainian national guardsmen.
Alexander Lukashevich, a spokesman for the Russian Foreign Ministry, denounced the guardsmen as "ultranationalists ... who stained themselves with the blood of women, children and the elderly during their punitive operations".
Although Interior Minister Arsen Avakov had said the guardsman units could include the Azov Battalion, a far-right formation notorious for using an insignia used by many military units in Nazi Germany, US Embassy spokesman James Hallock said Azov fighters would not be among those trained.
Al Jazeera's Rory Challands, reporting from Moscow, said Russia would be watching closely "exactly who is being trained". Challands said if irregular groups such as Azov were among those trained, "it will give the Russians a very strong case that the US is training fascists and neo-Nazis".
Though fighting has diminished substantially since a February ceasefire deal was signed in Minsk, Belarus, clashes continue and each side accuses the other of wanting to resume the conflict.
Lukashevich also said the US troops' presence violated the section of the Minsk agreement that banned foreign "armed formations" from Ukraine.
A 'self-evident escalation'
On Friday, the UN Human Rights Commissioner's office said at least 6,116 people have been killed since the fighting broke out a year ago between the government of Ukraine and pro-Russian separatists.
National guard units, many of which began as volunteer groups, have been an important part of Ukrainian forces' fighting against the separatists.
Two national guard units, working on weeklong rotations, are holding the town of Shyrokyne, currently the most fraught location in the east. Shyrokyne is just east of the major port city of Mariupol, which Ukraine fears rebels aim to seize to establish a land corridor between the Russian mainland and the Russia-annexed Crimean peninsula.
Dmitry Peskov, a spokesman for Russian President Vladimir Putin, also criticised the American move.
"The participation of instructors and specialists from a third country on the territory of Ukraine, where an unresolved intra-Ukrainian conflict remains, where problems persist in carrying out the Minsk agreement, is far from helping resolve the conflict. To the contrary, it enables destabilising the situation," Peskov said.
Denis Pushilin, the envoy for the Ukrainian rebels, said the exercises were "a self-evident escalation".
Al Jazeera's Challands said it could be assumed "that the Kremlin is more worried about this than they're letting on".
"They will see this as part of a continuation of NATO encroachment on Russia and they have always been against the joining of Ukraine under the NATO security umbrella," Challands said.
Source: Al Jazeera and agencies