Retreating pro-Russian insurgents dug today in Ukraine's sprawling industrial hub of Donetsk after government forces scored a string of morale-boosting victories in the bloody battle for the future of the country.

The eastern home of one million mostly Russian speakers has been flooded with convoys carrying hundreds of fighters and scores of anti-aircraft guns from five smaller surrounding cities where Ukrainian flags were flying for the first time in three months.

The rebels erected checkpoints along the main roads leading into Donetsk while the centre of the riverbank city itself saw several restaurants and shops shutter their doors.

The separatists' "tactical retreat" began on Saturday with the fall of their symbolic bastion Slavyansk and continued overnight into Sunday until government forces had reached the very gates of the region's main metropolis.

Ukrainian National Security and Defence Council deputy head Mykhailo Koval said on Monday that soldiers now intended to complete a "full blockade" of both Donetsk and the neighbouring separatist stronghold Lugansk -- both capitals of their own "People's Republics".

Koval said the containment of the cities would be followed by "corresponding measures that will force the separatists, the bandits to lay down their arms".

His carefully-worded comments underscored the dilemma facing Western-backed President Petro Poroshenko as he seeks to fulful his May 25 election promise to quickly end Ukraine's worst crisis since the ex-Soviet state's independence in 1991.

The conflict has claimed the lives of nearly 500 people and displaced tens of thousands across an economically-vital region that has long viewed the more nationalistic west of Ukraine and Kiev with a mixture of hostility and mistrust.

A Ukrainian shelling campaign of either Donetsk or Lugansk of the type that pulverised parts of Slavyansk would seem unimaginable because of both the inevitable toll and the high probability of an already-fuming Kremlin responding by sending in its troops.

It would also risk throwing the strategic nation of some 45 million into an all-out civil war on the European Union's eastern edge that would pit Russia against Western powers in a standoff of a scale not even witnessed during the Cold War.