NEW YORK (NYTIMES) - Russia has withdrawn most of its helicopters from a strategic airport in Kherson, in southern Ukraine, according to satellite images analysed by The New York Times, in what experts said could be a telltale sign of Russian military setbacks in the south of the country.

The removal of the equipment from the airport, evidenced by images captured by the space imaging company Planet Labs over six days, comes as the Ukrainian army is pressing to retake lost territory in the Kherson region.

Kherson, a shipbuilding centre east of Odessa on the Black Sea, was the first major city to be overwhelmed by Russian forces in the early days of the war. But Russia has failed to overtake the region as a whole.

Control over Kherson is essential to any effort to dominate the south broadly. The region, which lies just north of the Russian-controlled Crimean peninsula, stretches from the Black Sea coastline to the mouth of the Dnieper River.

Last week, Ukrainian forces attacked the Kherson airport, inflicting considerable damage to Russian equipment, which was clearly visible in satellite images and video of the aftermath.

An image taken on Monday (March 21) shows that previously visible aircraft had been removed, although Russian ground troops appear to still control the airport.

"The Ukrainian attack itself shows the vulnerability of the position, and the Russians may have decided that it's unwise to keep expensive aircraft parked there," Mr Frederick Kagan, the director of the Critical Threats project at the American Enterprise Institute, wrote in an email.

He said Russian forces appeared to have given up, at least for the moment, on taking Mykolaiv, a strategic city located on an inlet of the Black Sea, as well as other critical areas in southern Ukraine, like Odessa, a major economic and cultural centre.

"Kherson airfield is most useful for those operations," he added.

A video posted to Twitter on March 18, and verified by The Times, shows Russian vehicles towing helicopters away from the airport through a town about 25 miles (40km) to the south-east.

Mr Mason Clark, a senior analyst and Russia team leader at the Institute for the Study of War, wrote in an email that the Russians on the whole were pulling back manned aircraft as a result of losses sustained against Ukrainian forces.

Mr Clark said the Russian air operations may also have been impacted by casualties of crew and mechanical support staff and exhaustion.