Russia warned the United States yesterday against supplying arms to Ukrainian forces fighting pro-Russian separatists in eastern Ukraine, hours before US Vice-President Joe Biden arrived in Kiev.

Ukraine accused President Vladimir Putin of treating its territory like a “playing field”, trying to unleash a full-scale war that would pose a broader threat to Nato countries.

Russian Foreign Ministry spokesman Alexander Lukashevich said in Moscow that a US official’s suggestion Washington should consider sending arms to Ukraine, where pro-Russian rebels have been fighting government forces since April, sent a “very serious signal”.

Lukashevich cautioned against “a major change in policy of the US administration in regard to the conflict” in Ukraine.

“That would be a direct violation of agreements reached, including agreements reached with the participation of the United States,” he said.

Washington backs Kiev in its struggle against the pro-Russian separatists in two eastern regions and has imposed sanctions on Russia over its policies.

Moscow supports the separatists but denies it is backing the rebels with arms and troops in a conflict which the UN says has killed more than 4,300 since mid-April.

Swiss diplomat Heidi Tagliavini, the Organization for Security and Cooperation in Europe’s envoy to a Trilateral Contact Group on Ukraine, cited reports of new military buildup.

US President Barack Obama’s choice to fill the number two spot at the State Department, Anthony Blinken, told a congressional hearing on Wednesday:

“I believe that, given the serious Russian violations of the agreement that they signed...that one thing that could hopefully get them to think twice and deter them from further action is strengthening the capacity of the Ukrainian forces, including with defensive lethal equipment.”