A planned meeting between the leaders of France, Germany, Russia and Ukraine aimed at finally putting an end to the war in east Ukraine appeared uncertain on Sunday, as violence in the region spiked.

A spokesman for German chancellor, Angela Merkel, said she was not ready to travel to Kazakhstan for a planned summit on Thursday and would not go unless she saw evidence of real commitment from Russia to implement an earlier ceasefire plan. The comments came after Merkel spoke with the Russian president, Vladimir Putin, by telephone on Sunday.

Hopes for the Kazakhstan summit were boosted when the French president, François Hollande, spoke with Putin earlier this month, and called for an end to European sanctions against Russia and renewed discussions over east Ukraine, in an attempt to put last September’s Minsk peace agreements into practice. However, with Hollande’s mind now on domestic issues, Thursday’s meeting seems unlikely to take place. However, foreign ministers from the four countries are due to meet in Berlin on Monday.

The Ukrainian president, Petro Poroshenko, also spoke with Merkel on Saturday, and admitted a “drastic escalation of the conflict” in east Ukraine during recent days.

Two Ukrainian soldiers were killed and 20 wounded on Saturday, according to Ukrainian authorities, while three civilians including a 14-year-old girl were killed by shelling in the Luhansk region on the same day, local police reported.

Both Kiev and Moscow have an interest in negotiations: the Ukrainian forces have found it impossible to win control back militarily against separatist forces with Russian backing, while Moscow is facing an economic downturn and does not want to prop up the Donbass region financially.

Visiting Berlin last week, Ukraine’s prime minister, Arseniy Yatseniuk, said the effect of Russian sanctions have made Russia more willing to sit round the table, but insisted Russia was yet to fulfil three commitments it agreed to in Minsk: to give Ukraine control over its border, stop the shipment of weapons to separatists, and withdraw its troops and agents.

For its part, Russia denies ever sending troops or weapons to Ukraine, despite evidence to the contrary, and says it is Kiev that is failing to implement the Minsk accords.

Yatseniuk said he often hears that any agreement should allow Putin to save face. “It sounds good if there is a face to save, but it’s harder to do if the face is wearing a cynical mask,” he said.

Around 60% of Ukrainians believe Kiev should fight to win the territory back, according to a recent poll, and Poroshenko has been ramping up the war footing, which experts suggest is “flexing muscles” to gain a stronger position ahead of negotiations.

The president was photographed shooting a machine gun at a training range near the front lines last week, and presented the army with two new war planes, armoured vehicles and mortars. At a press conference in late December, Poroshenko said he firmly believed a peaceful solution to Donbass was the best option, but the imposition of martial law in the region remained on the table if a solution could not be reached.

Ukraine is planning to conscript around 200,000 young men for military service in 2015, many of whom will replace those currently serving in the Donetsk and Luhansk regions.

On the ground, fighting has broken out between different rebel groups, with one commander known as Batman killed in an ambush by forces belonging to the Luhansk People’s Republic, the quasi-state he supposedly served. Publicly, however, the rebels still insist there can be no negotiations on coming back under Kiev’s rule.

The Guardian caught up with Igor Strelkov, the best known of the rebel leaders, at a concert of patriotic pop songs in a town outside Moscow on Thursday evening. Strelkov left the region in August, apparently after pressure from Moscow due to his controversial tactics and emerging cult status.

“The war will carry on,” Strelkov said. “Ukraine is not serious, they have not even offered Russian as a second state language, and there is no way that Putin and Poroshenko can agree on anything.”

When asked whether he intended to return to the battlefield, he frowned and said, “That does not depend on me,” before cutting the conversation short.

A source close to the separatist movement said there was a great deal of uncertainty among the rebels about what would happen, but that it was clear a decisive period had begun.

“It’s very difficult to make any predictions, but the next two or three weeks will decide everything, one way or the other,” said the source.

Valentyna Romanova, a Ukrainian political analyst, said if the presidents do get together around one table, the talks could prove more fruitful that last year’s agreements, due to their multi-level format, with officials from all countries working out the specifics of a deal on paper first, followed by foreign ministers visiting Berlin this week.

“Then if the country leaders put their signatures on the agreement in Astana it will have more power than the Minsk agreement, which wasn’t signed by either Poroshenko or Putin,” she said.