WASHINGTON — Buoyed by support from its NATO allies, Turkey escalated its warnings against Syria on Tuesday, even as some American and allied officials privately raised questions about whether the Turkish warplane shot down by Syrian air defenses — provoking the denunciation — had been on a spy mission.

In response to the downing, Prime Minister Recep Tayyip Erdogan of Turkey warned Syrian forces to stay clear of their troubled border or face a military response to any perceived threat. Mr. Erdogan’s bellicose tone came as ambassadors from the Atlantic alliance, seeking to avoid a wider conflict, held emergency talks in Brussels at Turkey’s behest.

After the meeting, the NATO secretary general, Anders Fogh Rasmussen, said the alliance considered Syria’s actions in shooting down the Turkish warplane to be “unacceptable.”

While the American and allied officials emphasized that some intelligence reports flowing in since the downing last Friday were murky and often conflicting, they said a preliminary analysis of the available data suggested that there may have been more to the aircraft’s mission than just a routine training exercise to test Turkey’s air defenses.