Social media images from Wednesday showed Ukrainian troops in at least one village in the eastern Luhansk area, after crossing from the neighboring Donetsk region.
One photograph showed a Ukrainian unit kneeling and standing around a road sign at the village of Hrekivka, just inside Luhansk region.
It is the first time since the beginning of the conflict in March that Ukrainian troops have advanced into Luhansk.
More on Ukraine's advances: All of Luhansk region is claimed as Russian territory by the Kremlin, following its forcible annexation. But in recent days Ukrainian forces have been approaching the region from several directions, building on their successful offenses in Kharkiv and Donetsk.
Social media video also showed Ukrainian troops in the town of Terny in Donetsk region, about 20 kilometers (about 12 miles) from the town of Kreminna in Luhansk, which analysts believe is a critical defensive line for the Russians now that they have lost ground in both Donetsk and Kharkiv regions.
The Ukrainian advances in the northeast come within days of the so-called referendums held by pro-Russian local authorities that led to the annexation by Moscow of Donetsk and Luhansk as well as much of Zaporizhzhia and Kherson. Since the annexation measures were approved by Russian President Vladimir Putin last Friday, Russian forces have lost hundreds of square kilometers of territory in Donetsk and Kherson.