Russia has resumed a military build-up near the Ukrainian border, the NATO secretary-general has said, calling the deployments "a very regrettable step backward".

Anders Fogh Rasmussen said on Thursday that Moscow appeared bent on using its military to further intimidate Ukraine.

"I can confirm that we now see a new Russian military build-up - at least a few thousand more Russian troops deployed to the Ukrainian border, and we see troop manoeuvres in the neighbourhood of Ukraine," he said.

"If they're deployed to seal the border and stop the flow of weapons and fighters that would be a positive step. But that's not what we're seeing."

Rasmussen said "the international community would have to respond firmly if Russia were to intervene further".

Jack Lew, the US treasury secretary, said on Thursday that Russia must back the peace plan instead of destabilising the situation.

"We continue to urge Russia to work with Ukraine to reach a negotiated resolution to the current situation. But if Russia is unwilling to reverse course, the United States and the international community is prepared to impose additional cost," Lew said at a news conference in Berlin.

NATO estimated at one point there were up to 40,000 Russian forces deployed near the border with Ukraine, but reported last month that many of the soldiers and their equipment had been pulled back.

EU deal

Petro Poroshenko, Ukriane's president, said on Thursday he would sign the "association agreement" with the EU on June 27.

He added that his new foreign minister, Pavlo Klimkin, would be sent to Luxembourg next week to lay out a peace plan for the country's restive east to EU ministers.

Referring to plans to sign the EU agreement, Poroshenko said: "That for which we have waited for so long will take place next week."

Meanwhile, Denis Pushilin, the leader of the self-proclaimed Donetsk People's Republic, said on Thursday that the pro-Russian fighters were significantly weaker than the Ukrainian army.

Speaking in Moscow, Pushilin said pro-Russian fighters were losing most of their battles with Ukrainian troops.

Pushilin also said the troops had another major advantage over pro-Russia separatists - the support of the West.