The commission said there will be “technical level as well as … political level consultations” to “discuss implementation aspects” of an EU-Ukraine free trade treaty, which is to be signed in Brussels on 27 June.

Barroso first spoke to Ukraine President Petro Poroshenko to obtain his blessing for the initiative.

Ukraine diplomatic sources say the consultations are to begin after 27 June. “We are not speaking here about asking Russia whether we should sign or not sign the treaty because that decision is our sovereign right”, a Ukrainian contact told EUobserver.

A commission source said the talks would not discuss changing the content of the treaty, but only how to implement it in a way which does not harm Russia's trade interests.

The treaty would mark a shift in international relations by legally binding Ukraine to integrate with the EU and to stay out of Russia's Eurasian Union.

Behind the scenes, there is little faith the Barroso initiative will make Putin change course.

But EU sources said it will help to counter Russian propaganda that Moscow is being kept in the dark on EU-Ukraine developments.

The day Barroso spoke with Putin, the US State Department said it has evidence that Russian tanks, artillery, and rocket launchers are crossing into Ukraine.

The following morning pro-Russia fighters shot down a Ukrainian military plane, killing 49 servicemen, while Ukrainian security services found a bomb near Poroshenko's building in Kiev.

The commission source also told EUobserver it has completed work on a package of economic sanctions to be imposed on Russia if it does not de-escalate by the end of the month. “The work has been done,” he said.

The EU and US decision not to implement the sanctions already is causing frustration in Ukraine.

“It is very difficult to understand why we are still hearing threats that ‘a failure by Russia to de-escalate this situation will lead to additional costs.’ Russia has not just failed to de-escalate the situation. It is actively escalating it,” Halya Conyash from KHPG, a Ukrainian NGO, wrote on Saturday.

The Ukrainian diplomatic source said: “For a long time, we’ve had Russian small arms, Russian special forces, and Russian intelligence officers pouring into our territory. Now we have tanks and rocket launchers. What needs to happen for the EU to realise that Putin isn’t keeping his word?”.

Will they, won’t they?

EU countries and Ukraine say they will sign the treaty on 27 June no matter what Russia does.

But Ukraine also says the EU decision to wait until that date is one reason why the conflict is getting worse.

“So long as there is a prolongation of the period of uncertainty, the situation will not de-escalate. If we had signed it already, then Russia would be forced to build new relations on the basis of a clear-cut position,” the Ukrainian diplomatic source added.

Ukraine’s caretaker PM Arseniy Yatsenyuk wanted to sign the pact on 21 March. But the EU opted to wait until after Ukraine’s presidential election on 25 May.

Poroshenko wanted to sign on 7 June, the day of his inauguration. But the EU said it was not ready in technical terms.

EU ambassadors will meet in Brussels this week with a view to adopting a formal decision to sign. Member states are currently in talks on the legal formula of the decision, after an EU court ruling on a similar treaty with the Philippines put in doubt the validity of the draft Ukraine formula.