As Ukraine's incursion into Russian territory enters its third week, key bridges have been coming under attack.
Over four days, three bridges in the Kursk region were either destroyed or damaged.
Analysts say these were part of critical supply lines for Russian troops defending the area.
And it will hinder Moscow's efforts to push back Kyiv's advance into western Russia, which is causing a major dilemma for President Vladimir Putin.
Ukraine claims its forces now control more than 1,250 square kilometres and 93 settlements across Kursk, which sits along its north-eastern border.
The incursion is the biggest invasion of Russia since World War II, and is changing the trajectory of the war.
But analysts say Ukraine still has a long way to go, and now needs to defend the territory it has gained.
Bridges under attack
The Seym River snakes westward through a large part of the Kursk region and continues to Ukraine.
Along the river, three main bridges provided the only routes for Russia to get supplies and reinforcements to and from the area.
According to the Ukrainian Air Force, Russian military bloggers and officials, they have all come under attack.
On Friday, the first of the bridges fell in the town of Glushkovo.
The Ukrainian Air Force posted a video on social media claiming to show the moment of impact.
Russian media and military bloggers claimed the bridge was struck by two volleys of missiles from a US-made High Mobility Artillery Rocket System (HIMARS).
Glushkovo is about 12km north of the Ukrainian border, and approximately 16km north-west of the main battle zone in Kursk.
The Russian MASH news outlet claimed on Telegram that part of the district had been cut off.
It said the civilian population could now only be evacuated by water.
In the nearby village of Zvannoye, a second bridge was bombed on Saturday, creating a large hole along the roadway, according to analysis by the US Institute for the Study of War.
There are conflicting reports about the extent of the damage and whether the bridge is still passable.
Zvannoye is located another 8km to the north-west of Glushkovo.
"Minus one more bridge!" the Ukrainian Air Force commander, Lieutenant General Mykola Oleshchuk, wrote on Telegram.
He said his forces had destroyed the two bridges to weaken enemy logistics.
Russia reported damage to a third bridge on Monday in the village of Karyzh.
Kyiv was yet to comment on the attack.
By taking out the last standing link in the area, analysts say Ukraine may be trying to encircle Russian troops in the area of about 700 sq km.
It is not clear how many Russian troops could potentially be trapped.
Ukrainian media and Russian military bloggers were reporting that the destruction of the three bridges had left about 80 Russian villages in the area without road access to points north and east.
Russia was reportedly building floating pontoon bridges to transfer its forces, and was attempting to evacuate civilians on small boats.
But pontoon bridges would be particularly vulnerable to HIMARS attacks and other air strikes.
HIMARS are one of Ukraine's most advanced US-provided systems, and have been integral in Kyiv's ability to challenge Russia.
Ukraine was only recently granted permission to use US-supplied weapons for limited strikes in Russian territory.
When asked about the weapons used in Kursk last week, the Pentagon said Ukraine was "taking actions to protect themselves from attacks that are coming from a region that are within the US policy".
What Ukraine is trying to achieve
Analysts say it appears Ukraine is attempting to isolate the stretch of Kursk territory which lies just west of the invasion zone.
The Ukrainian Center for Defense Strategies said the attacks "suggest that the objective of Ukrainian defence forces is to take control of a portion of Kursk Oblast south of the Seym".
Malcolm Davis, senior analyst in defence capability at the Australian Strategic Policy Institute, said the impact was intended to be felt beyond the Kursk region.
By targeting the bridges, Ukraine was cutting off logistical supplies to Russian forces deployed in Donetsk Oblast in eastern Ukraine.
The situation along the eastern frontline has been increasingly difficult for Ukraine under tremendous Russian pressure, Dr Davis said.
Russia wants control of all parts of Donetsk and neighbouring Luhansk, which together make up the Donbas industrial region.
"By cutting those bridges, it makes it very difficult for the Russians to send munition supplies to those forces," Dr Davis said.
"Basically, it's about reducing Russian combat capability through taking out supply lines.
"The Ukrainians want to weaken the capability of those forces, so they can eventually go back on the counter offensive in eastern Ukraine."
The incursion in Kursk hasn't calmed the fierce battle raging for the strategic eastern city of Pokrovsk, which was at threat of falling to Russia.
The head of the local military administration, Serhiy Dobriak, said Russian troops were about 10km from the outskirts of the city, a transport hub for Ukrainian forces.
Ukraine to start digging in
Ukraine launched its surprise strike on the Kursk region on August 6.
Over the weekend, Ukrainian President Volodymyr Zelenskyy said the military incursion aimed to create a "buffer zone", suggesting it could prevent attacks by Moscow from across the border.
Ukraine's top commander Oleksandr Syrskyi said his forces had pushed 28 to 35km into Russia, capturing 1,263 sq km of territory, including 93 settlements.
More than 121,000 people have been evacuated from nine border districts in the Kursk region, Russia's emergencies ministry said.
Dr Davis said when the Ukrainians first went into Kursk, the Russians were in complete disarray.
But they were now expected to start getting their defences organised.
"What's happening now is the Ukrainian advance in Kursk is culminating, which means that they won't be able to necessarily advance that much further," he said.
"Ultimately, they will become fixed in place.
"They'll define a suitable defensive perimeter, pull back to that defensive perimeter, and defend that territory."
Rajan Menon, an international relations professor at The City College of New York, said the longer Ukraine could hold Russian territory, the better bargaining power it would have to find an end to the war.
If Kyiv could defend territory in Kursk, then Mr Zelenskyy "has cards to play at the table".
"The game has changed, if the Ukrainians are able to sustain this operation," he told ABC News Daily.
"If it can hold this territory, it can say, 'We're prepared to withdraw from it, if you withdraw from places x,y,z.'
"So, the run-up to any political settlement that would occur has changed drastically, providing the Ukrainians can hold onto this terrain."
Belarus boosts defences
Belarus has claimed Ukrainian troops were also edging towards its territory.
Belarus President Alexander Lukashenko announced he had ordered almost a third of the army to the 1,084-km border with Ukraine.
Although he did not specify troop numbers, Belarus' army numbers are about 60,000.
Mr Lukashenko — a loyal ally of Mr Putin — accused Kyiv of aggressive policies and of sending more than 120,000 of its troops to the frontier with Belarus.
Dr Davis said the move wasn't particularly significant, describing the Belarusian military was "not that capable or strong".
And it was unlikely Belarus would enter the war anytime soon.
"What it does is distract the Ukrainians, and forces the Ukrainians to keep some forces in place in that area, just in case," he said.
"Belarusians came across the border into Ukraine, they would certainly not come out of it well."
ABC/Wires
