A US think tank says Russia has dug new trenches near the border with Ukraine, as Ukrainian forces continue cross-border operations.

The Ukrainian forces launched large-scale cross-border offensives in the western Russian region of Kursk on August 6. They say they have so far taken control of about 1,000 square kilometers of the Russian territory.

The Institute for the Study of War in the United States has analyzed satellite imagery taken on Monday.

The institute says it shows "a newly dug series of field fortifications, including trenches and anti-vehicle ditches" along a highway around the town of Lgov in the Kursk region.

It also says that "Russian forces are concerned about potential continued and rapid Ukrainian mechanized northward advances," and they appear to be "particularly concerned about major highways."

The institute notes that the new fortifications are likely a part of Russian efforts to "preemptively safeguard important ground lines" of communications to inhibit Ukrainian maneuver.

The Ukrainian military announced on Wednesday that its forces, together with the nation's security and intelligence services, attacked four airfields in the Russian regions of Kursk, Voronezh and Nizhny Novgorod.

The military noted that Russia's army aviation, Sukhoi combat jets and other military aircraft are based at those airfields. They added that the main targets were fuel, lubricants and aviation munitions depots.