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Lviv, Ukraine (CNN) --
The war in eastern Ukraine appears to be entering a critical phase, as Russia sends in more combat units and Ukrainian forces try to hold lines battered after weeks of shelling and rocket fire.
The Ukrainian military on Wednesday acknowledged losing territory in two different areas of the long front lines they are defending, a reflection perhaps of the sheer volume of Russian forces being deployed, as well as improved Russian tactics.
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Essentially, the Ukrainian forces are defending three sides of a slowly shrinking group of fighting forces in the east, with their only supply lines coming from the west.
The risk - if the Russians make progress - is that some of the best units in the Ukrainian army, which lack protection from the air, could be cut off.
The Ukrainian military said on Wednesday that Russia transferred two more battalion tactical groups of the 76th Airborne Assault Division from the Russian city of Belgorod to the Izium area inside Ukraine, as well as cruise missile units.
The Kremlin's stated goal is to secure all of the Luhansk and Donetsk regions, and the Russian military is also seeking a permanent land corridor connecting Russia's territory with Crimea along the Ukrainian coast.
The Russian modus operandi seems similar regardless of location: days and sometimes weeks of heavy artillery, missiles and airstrikes that pulverize towns and cities, followed by the slow advance of the army.
In the Donetsk region, the Russian focus is on taking two major cities: Sloviansk and Kramatorsk, both of which were briefly held by Russian-backed separatists in 2014.
Russian units attempt to advance toward these centers from three directions.
Forward military units are likely to be within 10 miles of Sloviansk, and images emerged on Wednesday of a bridge southeast of the city that had been blown up, possibly a defensive move by Ukrainian units fighting a tactical retreat.
The Russian Defense Ministry in its recent statements has suggested that the offensive is accelerating, supported from the air.
On Wednesday, he claimed that 50 Ukrainian formations had been attacked by his fighter jets in the previous 24 hours.
The Institute for the Study of War (ISW) in Washington says Russian tactics appear to have improved.
“Russian troops are pushing down multiple roughly parallel paths at supporting distance from each other, allowing them to bring more combat power than their previous practice,” ISW says in its latest analysis.
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South of Izium, Russian forces are advancing along three routes.
It is as if the Russian commanders had assimilated the lessons of Kyiv, when a single Russian column sat north of the capital for weeks.
ISW says another factor in their advance may be that north of the Donbas region the Ukrainians do not have fixed defensive positions, as they have in Donetsk and Luhansk along the "line of contact" since 2014. Those defenses mean that the Russians are still fighting, despite weeks of missile and artillery attacks, to take places like Avdiivka, near the city of Donetsk.
Similarly, Ukrainians have held out in the city of Severodonetsk in the Luhansk region despite heavy shelling witnessed by a CNN crew on Wednesday.
But places like Severodonetsk and nearby Rubizhne are almost completely bankrupt and at the end of long, tenuous supply lines for the Ukrainians.
In turn, there are growing signs that the Ukrainians are targeting Russia's long supply lines.
There have been more unexplained "incidents" far beyond their front lines, as fuel and ammunition depots in western Russia suddenly exploded in the middle of the night.
A fuel depot near the Ukrainian border burned down in the early hours of Wednesday, as did another near Belgorod 24 hours earlier.
Ukraine has not directly claimed responsibility for these events, but presidential adviser Myhailo Podolyak said cryptically on Wednesday: "In these Russian regions, the large fuel depots that provide fuel for the armored vehicles of the Russian army are periodically burned and the depots of Ammunition explodes. For various reasons."
These rear positions represent obvious and necessary targets for the Ukrainians to attempt to complicate Russia's ability to sustain its ambitious offensive.
The Ukrainians have also shown that they can plan and execute successful counterattacks that harass the Russian flanks, and will look for new opportunities as the Russians try to advance.
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the south front
The Russians have also increased the pace of military operations in the south.
Over the past week, villages in the Zaporizhia region have come under heavy shelling, and the city of Kryvyi Rih now appears to be on the Russians' radar.
That is at least the working assumption of Ukrainian officials.
The Ukrainian army's Southern Command said on Wednesday that Russian units were regrouping and conducting aerial reconnaissance as they tried to improve their tactical positions.
A large area north of Mykolaiv has seen days of heavy fighting, with a Ukrainian soldier commenting on video that he saw a large number of Russian armored personnel carriers.
In the past two days, a backwater in southwestern Ukraine has also been caught up in the conflict, following two missile attacks on the only bridge connecting it to Odessa and the rest of the country.
This area is particularly vulnerable given the presence of some 1,500 Russian troops in Transnistria, a breakaway region from neighboring Moldova.
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The destruction of the bridge followed two unexplained sabotage attacks in Transnistria.
Ukrainians see these incidents as provocations planned by the Russians, a possible pretext to enter the region west of the Dniester River, which borders Romania.
Last week, a top Russian general said that Russia intended to establish "full control" over southern Ukraine during the second phase of its invasion, adding that doing so would give its forces access to Transnistria.
In general, it seems that after abandoning the plans to subjugate Kyiv, the Russian forces are trying to stretch their opponents along the front lines that now stretch for hundreds of kilometers: from the east of Ukraine to the south deep.
At the same time, they are attacking infrastructure far behind these front lines, from railway power grids in western Ukraine to fuel and weapons stockpiles across the country.
In the short term, it appears that the Russian command wants to deliver tangible results to the Kremlin in time for the Victory Day parades on May 9.
In the longer term, questions remain about President Vladimir Putin's ultimate goals in Ukraine.
The Ukrainians will need every last piece of promised Western weaponry to withstand what is now an attack, as well as nimble tactics to ensure units are not surrounded.
Russian invasion of Ukraine
