Russia on Friday warned that the arrival of US paratroopers in Ukraine to train its forces fighting pro-Russian rebels could reignite the conflict, leading to mass bloodshed.

The arrival of hundreds of US paratroopers in war-torn Ukraine causes "serious concern," the Russian foreign ministry said in a statement.

It warned that the US training programme was a step towards Washington supplying weaponry to the Ukrainians fighting pro-Russian separatists in the country's Donbass region.

"Washington's encouraging attitude to (Kiev's) revanchist plans carries a risk of reigniting mass bloodshed in our neighbouring country," Moscow said.

"It's obvious that American soldiers on Ukrainian soil will not bring it peace."

President Vladimir Putin's spokesman Dmitry Peskov said earlier Friday that the arriving US troops "can seriously destabilise the situation."

Moscow blasted the US troops' mission to instruct the National Guard of Ukraine, a reservist force, for training those "who have stained themselves with the blood of women, children and old men in punitive operations in Donbass."

"What will the overseas military specialists teach them -- how to keep on killing those who speak Russian?" the ministry asked in a strongly-worded statement.

It argued that the military training programme was the "first step towards supplying Ukraine with the modern American weapons that Kiev's party of war so desires."

US President Barack Obama is under pressure from lawmakers and military officials to send weapons to Ukraine to help it fight the insurgency. but has so far held back.

Some of his European allies including Germany have warned that sending arms would cause the bloodshed to escalate.

Moscow argued that the arrival of US troops was a "clear breach" of the terms of a truce reached in the Belarussian capital of Minsk in February that called for the withdrawal of foreign troops from Ukraine.

Kiev says Russia has sent thousands of soldiers across the border in eastern Ukraine to support the rebellion, charges Moscow denies.

The ceasefire deal aimed to end a year of fighting that has killed more than 6,000 people, but fighting flared up again this month.