The Financial Times reports Western intelligence shows Russia is amassing aircraft close to the border with Ukraine. It said the move indicates Moscow is preparing to bolster its faltering land offensive.

The paper says intelligence shared among NATO allies shows Russia is assembling both fixed-wing and rotary aircraft.

The article notes that Russia has used its air forces sparingly, relying instead on long-range missiles, artillery and land-based troops.

Western analysts have speculated this could reflect a fear in Moscow that Ukraine's air defense systems posed a threat to Russian aircraft, or because its air force was in poor condition.

But a senior NATO diplomat is quoted as saying that more than 80 percent of Russia's air force probably is "safe and available," and that they are expecting the Russians to be "preparing to launch an air campaign."

A senior US administration official also reportedly said, "Russian land forces are pretty depleted so it's the best indication that they will turn this into an air fight."

The official added, "If the Ukrainians are going to survive ... they need to have as many air defence capabilities and as much ammunition ... as possible."