Warsaw, Sept. 11 -- Polish Prime Minister Donald Tusk pledged on Thursday to push ahead with a "great modernisation programme" for his country's military, a day after Russian drones crossed into Poland and forced the NATO alliance to confront a potential threat in its airspace for the first time.

European officials described Wednesday's incursion, which occurred during a wave of unrelenting Russian strikes on Ukraine, as a deliberate provocation. It deepened longstanding fears that the three-year war between Poland's neighbours could precipitate a wider conflict. US-led efforts to steer Moscow and Kyiv toward a peace settlement have so far failed to get traction.

The Polish Air Navigation Agency announced Thursday morning that Poland wa...