- Russia said it is fighting a cross-border incursion by Ukraine in its Kursk region.
- Experts said Ukraine made it more than six miles into Russia, past significant defenses.
Ukraine managed to break through at least two Russian defensive lines and get past a stronghold in its unprecedented attacking on Russia's own territory, experts said.
The Institute for the Study of War gave its assessment of the surprise offensive in an update on Wednesday.
"The current confirmed extent and location of Ukrainian advances in Kursk Oblast indicate that Ukrainian forces have penetrated at least two Russian defensive lines and a stronghold," it said. It didn't give a name or location of the stronghold.
Russian officials acknowledged the operation on Tuesday, saying they were fighting a cross-border attack.
Valery Gerasimov, Chief of the General Staff of Russia's Armed Forces, said on Wednesday that up to 1,000 Russian soldiers were involved in a fightback.
Ukraine has not officially acknowledged making the attack, though some officials have mentioned it indirectly.
The offensive is extremely unusual — Ukraine has fought Russia almost exclusively on its own territory since Russia mounted a full-scale invasion in February 2022.
The ISW said on Wednesday that Ukrainian forces made confirmed advances of up to 6.2 miles into the region, citing geolocated footage of Ukrainian armored vehicles.
It is not clear how strong Russia's defensive lines were.
Russia may not have been ready for such an attack, given how rare it is.
Ukraine's counterattacks in Russia have usually involved long-range weapons like drones or Western-provided missiles rather than ground forces.
Some Russian military bloggers — major sources of news in Russia's repressed media— were angry that Russia seemed so ill-prepared, The Moscow Times reported.
The US, one of Ukraine's biggest allies, also seemed caught off-guard.
A US State Department spokesperson, Matthew Miller, said Wednesday that the US hadn't got any advanced warning.
He said "It's not unusual for Ukrainians not to notify their exact tactics before they execute them" and noted that Ukraine didn't seem to be violating any of its agreements with the US.
American officials have insisted that Ukraine not use the long-range weapons it donated against targets deep inside Russia.
Ukrainian officials and analysts have chafed at that restriction, saying it gives an advantage to Russia.
Russian President Vladimir Putin responded to Ukraine's advance by calling it a "large-scale provocation."
The US wasn't very sympathetic. Miller responded, saying "it's a little bit rich them calling it a provocation given Russia violated Ukraine's territorial integrity and sovereignty" first.