Poland had started to bolster its air defenses long before a stray missile landed just inside its border on Tuesday, but a robust shield for the skies along NATO's eastern flank is still a long way off after decades of neglect following the Cold War.

The missile that hit Poland appears to have been fired by Ukraine's air defenses rather than a Russian strike, NATO chief Jens Stoltenberg said on Wednesday.

But while it may have been a technical error that can happen in any conflict, the incident underlines the urgent need for NATO to plug gaps in its defenses because even mistakes like this could lead to a dangerous escalation.