EVEN a few days ago the signs of an impending showdown were there.

Driving into Sloviansk we saw armed pro-Russian militiamen busy reinforcing the checkpoints that guard the entrance to this, the most contested hotbed in eastern Ukraine's uprising.

Anyone venturing into the ­dilapidated industrial community could be under no illusions that, should it come down to a fight, those controlling the town would not give up easily. That much was made clear by Sloviansk's self-appointed separatist mayor, Vyacheslav Ponomaryov.

"We will make Stalingrad out of this town," he boasted, referring to the Soviet Union's defence of the Russian city in the Second World War.

Yesterday, it was more a series of probing attacks by the Ukrainian forces, perhaps to measure the degree of resistance in any full-out assault to retake this town of 130,000 people. What is almost certain is that they were testing Moscow's mettle and gauging Russian President Vladimir Putin's response.

While militarily limited, the strikes rammed home the ­Ukrainian government's message to Moscow that it will no longer tolerate the presence of separatist militants.

The first indications that a ­military push was under way in the last 48 hours came when the ­interior ministry announced it had "liberated" the resort town of Svyatohirsk "from unlawful armed groups."

This was followed in the early hours of yesterday by another announcement that the City Hall in Mariupol had been cleared of separatists. Yulia Lasazan, a spokeswoman for Mariupol's police department, said 30 masked men with baseball bats stormed the building and started beating the pro-Russian protesters.

Sloviansk is likely to prove a much tougher nut to crack. During my time in the town it was obvious that the separatist's defences were unlike any I had seen in other contested places, including Donetsk itself.

In Sloviansk's back streets, armoured personnel carriers sit protected behind sandbag emplacements. On its streets I watched as trucks ferried earth and sand being used to build bulwark military defences in the heart of the town.

With its narrow streets and dense civilian population it will prove problematic for Ukrainian forces, should they opt to fight their way into Sloviansk's centre. Yesterday, as the fighting erupted around the town, separatists rang church bells to sound a general alarm.

According to reports, the initial Ukrainian push was conducted as a commando raid against separatist controlled checkpoints to the north of the town. Up to three checkpoints were the scene of clashes and at least two pro-Russian separatists killed.

The Ukrainian storming of these checkpoints was backed by light armoured personnel carriers but indications are that Ukrainian forces rapidly withdrew after the gun battles.

A few days ago, while outside Sloviansk's City Hall building, I watched as a wall of sandbag emplacements was strengthened at its entrance and gunmen patrolled in front. As well as what could best be described as professional-looking gunmen, clearly disciplined and trained, there were also many volunteers who make up a militia around the locality.

"I have worked on checkpoints and I'm prepared to fight and take up weapons," insisted one 31-year-old man who would give only his first name: Artyom.

He said that up until recently he had worked in Russia as an electrician but had left his job and returned to Sloviansk as the separatist uprising gained momentum.

"I have had enough of this government in Kiev," he continued. "Donetsk region works hard for all of Ukraine but we receive nothing in return, so I would now like to see it part of Russia," Artyom told me.

Remarks from Moscow yesterday following the escalation around Sloviansk will no doubt be music to the ears of Artyom and his comrades who are prepared to defend the town.

Almost immediately, President Putin warned of "consequences" and Russia ordered new military exercises on its border with Ukraine.

Russian Defence Minister Sergei Shoigu said Moscow had been "forced to react" after the Ukrainian commando strike north of Sloviansk.

Escalating the ongoing brinkmanship even further, Ukraine's acting President, Olexander Turchynov, called on Russia to pull back its troops from the border and end what he called its "blackmail", adding that Kiev would not yield to "Russian-backed terrorists".

Last night Sloviansk was said to be calm, but few in the town and in eastern Ukraine believe the respite will last.

This crisis has just entered a new and even more volatile phase, the repercussions of which will be revealed only in the coming days.