The United States and its European allies are putting back on the table tighter sanctions on Russia after attacks by Moscow-backed rebels effectively ended a fragile, five-month truce in eastern Ukraine.

Reuters reported American and European officials accuse Russia of supporting the militants financially and logistically, as well as militarily with troops on the ground. Diplomats from NATO countries are set to meet with their Ukrainian counterpart at an emergency meeting in Brussels, where they will decide on how to respond to the end of the ceasefire.

Russian President Vladimir Putin, meanwhile, denied his country is militarily involved in Ukraine and accused that nation's central government of prolonging the conflict by refusing to negotiate with the militants. He said Kyiv's forces were aided by a "foreign NATO legion."

During his visit to India, President Barack Obama expressed deep concern over the renewed fighting and said he would seek to increase pressure on Russia, the Wall Street Journal reported.

"I will look at all additional options that are available to us short of a military confrontation in trying to address this issue," Obama said at a Sunday news conference in New Delhi.

There are doubts as to whether the West is unified enough to agree on substantial new sanctions against Russia. In fact, the European Union had already been thinking about conditions under which it might ease some of the restrictions placed on Moscow, which come with significant economic side effects for the bloc.

The EU's foreign-policy chief, Federica Mogherini, said on Saturday that more violence "would inevitably lead to a further, grave deterioration of relations between the EU and Russia."

Russian Foreign Minister Sergei Lavrov accused the West of whipping up anti-Russian hysteria to justify additional sanctions against Moscow, Reuters noted. Government spokesman Dmitry Peskov again denied any Russian involvement in the fighting in eastern Ukraine.

"Instead of increasing pressure on those who refuse to engage in dialogue to resolve the conflict peacefully, we see renewed talks about blackmailing Russia economically," Peskov said.