Ukraine's allies seem surprised by the assault, which had been surrounded in secrecy. (Representational)
Ukraine's incursion into Russia, which entered a third day Thursday, appears to be an unprecedented assault which experts say could aim to draw Russian resources away from other areas or to undermine morale.
Ukraine has maintained an unusual silence, with no one officially acknowledging that an operation was even taking place. Top officials have declined to comment, even on condition of anonymity.
Russia has given only scarce updates as it seeks to strike a balance between denouncing the escalation and avoiding panic.
Here are some key issues surrounding the cross-border attack:
What has happened?
At about 8.00am (0600 GMT) on Tuesday, Ukrainian forces launched an assault into Russia's southwestern Kursk region, according to the Russian army.
Russia says Ukraine deployed around 1,000 troops and more than two dozen armoured vehicles and tanks.
Russia accused Ukrainian forces of indiscriminately attacking civilian buildings, and rushed reinforcements, air and artillery firepower to repel the attack. A state of emergency has been declared in the region.
At least five civilians have been killed and 31 wounded since Tuesday, Russian officials said Wednesday.
Russia evacuated several thousand people from zones that have been shelled, while Ukraine announced a mandatory evacuation of around 6,000 people in the Sumy region, just across the border from Kursk.
Why is it special?
The assault seems unprecedented due to its reported scale, and because it appears to involve regular Ukrainian troops.
Fighters from Ukraine have made several brief incursions into Russia before. Some have been by units of Russians fighting in support of Kyiv -- the Russian Volunteer Corps and the Freedom of Russia Legion.
In mid-March, during the Russian elections, Ukrainian assault groups repeatedly tried to gain control of territory in the Belgorod region, officials had said.
Ukraine's allies seem surprised by the assault, which had been surrounded in secrecy. The United States says it has reached out to Kyiv for information.
Ukrainian military expert Sergiy Zgurets, who runs his own consultancy, told AFP that "the photos of the destruction of Russian and Ukrainian equipment, helicopters, the use of aviation, the use of artillery on both sides" indicated a significant military operation.
What gains have Ukrainian forces made?
Ukrainian troops have advanced up to 10 kilometres (six miles) into Russia, the US-based Institute for the Study of War said on Thursday.
The advance has centred on the logistical hub of Sudzha, a town of around 5,000 inhabitants about eight kilometres (five miles) from the Ukraine border.
Russian military bloggers, who have links to the army, also reported Kyiv had made significant advances with some reporting that Ukrainian soldiers control some parts of Sudzha.
What could be the aim of this raid?
The goals of the assault are unclear, this early in the operation and without any official comment from Ukrainian forces, but experts suggest several hypotheses.
Some said the incursion may seek to draw Russian reserves away from the nearby Kharkiv front in Ukraine, where Russia launched an offensive in May.
"For now, this is a limited operation, whose direct consequence will be to reduce the stability of the Russian group attacking/hovering over Kharkiv," Ukrainian military analyst Oleksiy Kopytko said on social media.
"Judging by the scale and intensity of the Ukrainian operation, sooner or later, the enemy will be forced to pull troops from other operational areas," Ukrainian military observer Kostiantyn Mashovets said on Facebook.
The assault could also have a psychological effect -- boosting the morale of Ukrainians who have seen their troops suffer a string of setbacks while trying to destabilise Russia.
"Another goal is to show Russian society what it feels like when your territory is occupied," Sergiy Solodkyy, deputy director at Kyiv's New Europe Centre think tank, told AFP.
Finally, some argue that if Ukraine could hold the territory, it would gain leverage against Russia in hypothetical peace negotiations.
Gaining an edge is crucial to Ukraine ahead of the US presidential election in November. Republican candidate Donald Trump is a Ukraine-sceptic.
"This is coercion to peace... ideally, Russia should withdraw troops," Solodkyy said.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)