WASHINGTON — Buoyed by support from its NATO allies, Turkey escalated its warnings against Syria on Tuesday, even as some US 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 mutual border or face a military response to any perceived threat. Erdogan's bellicose tone came as ambassadors from the NATO 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 US and allied officials emphasized that some intelligence reports flowing in since the downing last Friday were murky and often conflicting, they said an evolving analysis of the available data raised doubts about whether the Turks were being completely truthful in their account of the aircraft's mission.
They pointed to several unanswered questions about the episode, including why, given the tensions between the two countries, Turkey was flying an unarmed reconnaissance plane so close to the Syrian border, where the aircraft was struck, and whether it received any warnings to leave Syrian airspace.
Syria maintains that the plane was brought down by anti-aircraft fire well within its airspace. But Turkey says the plane was attacked over international waters after straying briefly into Syrian space during a training exercise to test its own air defenses.
US military and NATO officials said they were examining these claims as well as radar tracks and other classified information to better understand what happened.
But the officials said they were loath to publicly challenge an ally's version of the downing, which the White House and the State Department have condemned as unjustified and have cited as an example of the recklessness of the security forces loyal to President Bashar Assad of Syria.
"On a political level, NATO is taking the Turks at their word,'' said a senior US official who has reviewed classified reports of the incident.
One senior NATO diplomat said that even if the Turks were spying, it should not alter the international reaction."
When this happens between neighboring countries, you give a warning and then send up interceptors," said the official, who was not authorized to speak on the record. ''You don't just shoot down the plane.''
Turkey's deputy prime minister, Bulent Arinc, acknowledged on Monday that the aircraft — a two-seat RF-4E Phantom, an unarmed reconnaissance version of the F-4 fighter jet — is equipped for spying. But he strongly denied it was doing so on this particular mission. The RF-4E has the ability to gather high-resolution imagery about 60 miles from the target, aviation experts said.
The two countries have given sharply differing accounts of the downing. The Turkish foreign minister, Ahmet Davutoglu, has told state-owned TRT television that the aircraft was struck by anti-aircraft fire outside Syrian air space. ''Our plane was hit in international airspace,'' he said, ''13 nautical miles out of Syria, when Syrian territorial space is 12 miles.''
But the Syrian Foreign Ministry said Monday that the airplane was brought down by an anti-aircraft weapon with a range of less than 2 miles.