General Frank Gorenc, commander of US Air Forces in Europe, said he's concerned about Moscow's push to increase the quantity and quality of its aircraft and unmanned aircraft, Reuters reported.

"The advantage that we had from the air, I can honestly say, is shrinking," Gorenc said at the annual Air Force Association conference.

He went on to say that Moscow's investments in modernizing its air force and building formidable surface-to-air missile defenses are "alarming" moves.

US officials have often warned of China's developing military technology, particularly those aimed at defending against US stealth fighters and bombers, as well as increasingly capable surface-to-air missiles. But Gorenc says the concern does not just lie with China.

"This is not just a Pacific problem; it is as significant in Europe as it is anywhere else on the planet," he said. "I don't think it's controversial to say they've closed the gap in capability."