Diplomats from Russia and Ukraine agreed on Wednesday a dividing line from where both sides should pull back their heavy weapons, just hours after separatist forces deployed more arms and manpower to an emerging flashpoint in eastern Ukraine.

Germany’s foreign minister, who hosted a meeting of his counterparts from Russia, Ukraine and France, said the four parties had agreed that the demarcation line defined in the Minsk agreement of last year should form the basis for the withdrawal. Under the plan, Ukraine and the pro-Russian separatists would pull back their heavy arms nine miles on either side of the line, though there was no agreement on a withdrawal of all troops.

“Today we have finally agreed that the demarcation line mentioned in the Minsk agreement is the line from where the withdrawal of heavy weapons needs to take place now,” the German foreign minister, Frank-Walter Steinmeier, told reporters after the meeting in Berlin.

Steinmeier said the agreement had been “difficult work” and the talks, which follow a fruitless round of negotiations last week, were “testing the patience of all involved”. The parties also agreed that the contact group of Ukraine, Russia, and the OSCE should meet as soon as possible with the aim of laying further groundwork for a high-level meeting in Kazakhstan’s capital, Astana, aimed at reaching a long-lasting settlement.

Separately, the Russian foreign minister, Sergei Lavrov, said the “strong support” for the pullback was the meeting’s most important result. He said the foreign ministers did not discuss the sanctions that the west has imposed on Russia over the Ukraine crisis, saying: “The sanctions are not our problem, it is the problem of those who introduced them and now do not know how to extricate themselves.”

Earlier on Wednesday, Lavrov had urged measures to contain the unfolding unrest, but said nothing about the rebels surrendering territory they acquired in violation of a peace deal concluded in September in Minsk, Belarus. Ukraine says separatist forces that are backed by Russia have overstepped agreed-upon frontline boundaries between the warring sides by 190 square miles.

A fresh separatist advance is under way in an area north-west of Luhansk, the second-largest rebel-held city. The fighting is centered on two checkpoints along a strategic highway.

Ukraine’s defence ministry said one of those positions, Checkpoint 31, had been abandoned but that operations were under way to retake it.

The separatist forces appear well-poised to take the upper hand, however.

An Associated Press reporter saw nine Gvozdika self-propelled howitzers and six anti-tank cannons moving near the town of Perevalsk around midday. A rebel militiaman with the convoy who declined to give his name said the armament was heading in the direction of Checkpoint 31.

Along the same road, AP saw four Grad multiple rocket launchers accompanied by four trucks carrying ammunition and 15 pristine-looking tanks, also heading toward the checkpoint.

Ukraine and the west accuse Russia of providing material support to the separatists, which Moscow denies. The sheer amount of sophisticated heavy weaponry in the hands of the insurgents, however, is widely seen as overwhelming evidence of direct involvement by Russia.