Ukrainian government forces on Wednesday warned separatists in the eastern town of Donetsk that a plan was now in place to take back the territory they occupy, but defiant rebels reported a steady flow of new recruits who were ready to fight.
The Ukrainian military pushed the rebels out of their best-fortified stronghold in the town of Slaviansk on Saturday, but they have regrouped for a stand in Donetsk, a city of nearly a million people. Rebels also still control strategic buildings in Luhansk near the Russian border.
Separatists said on Tuesday that Igor Strelkov, a Russian military officer from Moscow who until the weekend led rebels in Slaviansk, had assumed command of the "defence of Donetsk".
President Petro Poroshenko has ruled out using air strikes and artillery that might endanger civilians and said on Tuesday night: "There will be no street fighting in Donetsk."
But the government says it has a plan to retake Donetsk and Luhansk and deliver a "nasty surprise" for the rebels.
Military spokesman Andriy Lysenko spelled out the threat on Wednesday, saying: "There's a plan to liberate Ukrainian territory from the terrorists, and it doesn't depend on the readiness or the unreadiness of Strelkov and his underlings to defend, as they call it, the Donbass."
But separatists in charge of a 'mobilization' centre for the self-proclaimed 'people's republic' said on Wednesday that recruitment of new fighters was continuing at a pace since Strelkov made an appeal for fresh recruits on Tuesday.
About 300 volunteers had come forward to join up since Tuesday, many more than the usual number of 25-30 people per day, separatists at the centre said.
Pro-Russian separatists have been fighting government forces in the Russian-speaking east since April in a conflict in which more than 200 Ukrainian troops have been killed as well as hundreds of civilians and rebels.
The conflict has driven relations between Russia and the ex-Soviet republic to an all-time low and sparked the worst crisis in Russia's relations with the West since the Cold War.
After a patchy performance at the start of the campaign, government forces have been re-invigorated by the Slaviansk victory and signs that rebel calls on Russia for help are now going unheeded.
Poroshenko referred back to those early days of the campaign when, he said, Ukrainian soldiers had refused to carry out orders and refused to fight, a time when "a bunch of provocateurs could stop a brigade of paratroopers or special forces".
"On July 7 the country had 320 km (200 miles) of open border (with Russia) ... A month has passed and now the army ensures the border is closed," he said.
German Chancellor Angela Merkel and French President Francois Hollande spoke by phone with Poroshenko on Wednesday, with the aim of restarting talks with separatists on a ceasefire, Merkel's spokesman said.
But hundreds of rebels are setting up barricades and digging in on the outskirts of Donetsk since pouring in from Slaviansk and nearby areas recaptured by the government.