An IRIS-T air defence system, video grab

Classified documents leaked in the press have revealed the Pentagon's concerns that Ukraine's frontline air defence may be depleted by the end of May, the New York Times reports.

According to US assessments, Ukrainian air defences designed to protect troops on the front line will "be completely reduced" by 23 May, leading to strains on the air defence network deeper into Ukrainian territory.

If that happens, the NYT adds, Russia may start using more air power to try to turn the tide on the frontline.

Unnamed senior Pentagon officials quoted in the report stressed that such a move would be a challenge for Ukraine, particularly if Russian aircraft were given freer rein.

Speaking to the New York Times, Yurii Ihnat, a spokesman for Ukraine’s Air Force Command, did not deny that Ukraine was suffering from depleted stocks of air defence munitions, but said that new systems delivered by Western partners could make up for the losses.

"The question is numbers," Mr Ihnat said. "To fully replace them, we need many systems, and I won’t tell you how many."

Ukraine has been introducing Western air defence systems since last year, the move expedited by Russian missile attacks on critical infrastructure.

For instance, Germany, the Netherlands, and the United States have pledged Patriot air defence systems, with the first group of Ukrainians troops having already finished the training.

In addition, France and Italy will deliver their SAMP/T system later in the spring.