Ukrainian forces have made rapid gains in recent days and have recaptured territory in the Russian-occupied Luhansk region of eastern Ukraine.
Russia invaded Ukraine on 24 February, surrounding the Ukrainian capital Kyiv. It also launched attacks in the south, east and north of the country.
In early April, Ukrainian forces retook large areas around Kyiv, after Russia abandoned its push towards the capital.
Russia has since been focusing its military operations in the south, east and north-east of Ukraine, seizing large areas. However, things changed dramatically at the start of September.
In a decisive offensive in the north-east, Ukraine drove back Russian forces. It claims to have regained 3,000 square kilometres (1,158 sq miles) of territory around the city of Kharkiv alone.
Its forces have also retaken territory in Luhansk, in eastern Ukraine. Since July, the region had been entirely occupied by Russia.
In total, Ukraine says it has retaken more than 8,000 sq km (3,088 sq miles) from Russian control in September - its most significant territorial gains since the war started.
The cities of Izyum and Kupiansk, which Ukraine says were retaken on 10 September, were both key supply hubs for the Russian forces. As such, these represent important strategic gains.
There is also a Ukrainian counter-attack around the Kherson region in the south of the country. However, Russian forces still hold about a fifth of the country.
The Institute for the Study of War (ISW) says Ukrainian troops have inflicted a "major operational defeat" on Russian forces.
Justin Bronk of the Royal United Services Institute said Russian positions in Kharkiv had suffered a "total collapse".
The Russian withdrawal was, he said, "certainly the most dramatic reverse that we've seen from the Russians since they retreated from Kyiv in April".