Russian forces are fortifying lines of defense in southern Ukraine, a new report says.

Russian forces are digging trenches and erecting barriers to strengthen their defenses against the possibility of a new Ukrainian offensive in the south following Moscow’s loss of a key city there, a research group said in a report published on Sunday.

Since Moscow ordered its troops to pull out of Kherson city earlier this month and retreat to the eastern bank of the Dnipro River, Russian forces have been working to fortify their defensive lines in the region, which is a gateway to the occupied Crimean Peninsula.

“The Russian defensive positions suggest that the Russian military leadership views the prospect of a Ukrainian counteroffensive across the Dnipro River as a serious threat,” the Institute for the Study of War, a research group based in Washington, said in the report. The report did not predict whether Ukrainian forces would attempt such a crossing.

Ukraine’s rivers have proved a formidable obstacle for both sides in the conflict, with attempts to cross them often putting troops in vulnerable positions. Analysts say that any concerted attempt by Ukrainian forces to cross the Dnipro River would most likely involve extensive preparation.

Russia began building a string of defensive positions in the eastern part of the Kherson region in October, the report said.Satellite images taken on Nov. 15 — four days after Ukrainian forces triumphantly swept into the city of Kherson — showed trenches and concrete anti-tank defenses, pyramid-shaped concrete blocks placed in rows, the report said, adding that Russian forces appeared to be to trying to protect key logistics routes.

At the same time, Russia has relocated some of its proxy administrators away from Nova Kakhovka, a town on the river’s eastern bank that is the site of a hydroelectric power plant — an apparent precaution, according to Ukraine’s National Resistance Center, a military organization that reports on Russian-occupied parts of the country. It was not possible to verify the report.

Ukrainian authorities have declined to give details of military operations in the south since the recapture of Kherson. Natalia Humeniuk, the spokeswoman for the Ukrainian military’s southern command, said on Monday that an operation to take control of the Kinburn Spit, a peninsula at the mouth of the Dnipro River was ongoing.

“This is very difficult work,” she told national television, citing the increasingly harsh weather conditions as winter approaches, as well as the location of the peninsula.

Ukraine’s attempts to win back territory seized in the nine months since Russia’s full-scale invasion began have come in waves. Some military analysts say that a relative pause is taking place in the south following Ukraine’s recapture of Kherson, as both sides attempt to regroup and refresh their forces while temperatures begin to plunge.