Western leaders roundly condemned the downing of a Turkish military fighter jet by Syria ahead of a NATO meeting on Tuesday on the issue.
U.S. Secretary of State Hillary Clinton on Sunday said she had spoken with Turkish Foreign Minister Ahmet Davutoglu about Friday's incident.
"The foreign minister briefed me on the specifics of the incident, including that the Syrian military shot its plane down without warning," Clinton said in a statement. "The United States condemns this brazen and unacceptable act in the strongest possible terms. It is yet another reflection of the Syrian authorities' callous disregard for international norms, human life and peace and security."
The Turkish Foreign Ministry said Sunday it considers the action to be a hostile act. Turkey delivered the message in a diplomatic note to the Syrian consulate in Istanbul, calling the incident "a hostile movement," ministry spokesman Selcuk Unal told CNN.
Turkish search-and-rescue teams found the wreckage of the fighter jet in the Mediterranean Sea on Sunday, about 1,300 meters (4,260 feet) underwater, he said.
They have not reached the wreck yet, he added. There was no word about survivors of the two-man crew.
"We will work with Turkey and other partners to hold the Assad regime accountable," Clinton said of Syrian President Bashar al-Assad. "Turkey has been a leader in the international community's effort to address the Syrian regime's violence against its own people."
The incident raises the temperature between the two regional powers significantly.
Syria gave no warning before shooting down the F-4 Phantom jet which strayed into its territory, Davutoglu said Sunday in an appearance filled with tough talk against Syria.
He accused Syria of spreading "disinformation" about the incident.
"They have created the impression that Syria felt like it was an act of aggression and they shot it down. ... from our perspective that's not the case," Davutoglu told reporters.
The plane in the Friday incident was unarmed, not sending hostile signals, and identifiable as Turkish, he said.
"You have to first send a caution, a warning," he said in the first detailed Turkish statement on the international incident. "If the warning doesn't work, you scramble your planes, you send a stronger signal, you force the plane to land. There wasn't enough time to do any of that in the time that our plane was in Syrian airspace."
"We have to question how it is that an unarmed, solo flight got this response from the Syrians," he said.
He said the fighter jet was in international airspace when it was fired upon.
It had strayed into Syrian territory in a "short, unintentional violation," but was notified by the Turkish side that it had crossed the line, and returned to international airspace, Davutoglu said.
Turkey will respond "decisively," but within international law, the foreign minister vowed, saying: "It's a fine line."
Turkey took its case to NATO, a spokeswoman for the alliance confirmed. Turkey, a key member of the group, is expected to make a presentation on the incident in Brussels Tuesday.
It called for the meeting under NATO's Article 4, which deals with what happens when the "territorial integrity, political independence or security of any of the Parties is threatened."
British Foreign Minister William Hague Sunday called the incident "outrageous" and said he condemned it wholeheatedly."
"The Assad regime should not make the mistake of believing that it can act with impunity. It will be held to account for its behavior," Hague said.
The plane was participating in a test of Turkey's national radar system, Davutoglu said.
Turkish boats and helicopters had been searching for the two-man crew inside Syrian waters, he said. He said the search-and-rescue mission was not a joint operation with Syria, but was being coordinated with Damascus because it is in their territory.
Davutoglu has also spoken with U.N. Secretary-General Ban Ki-moon and the foreign ministers of the United Kingdom, France, Russia and Iran, and the European Union foreign policy chief Catherine Ashton since the incident, Unal told CNN Saturday.
The Syrian military shot down the plane Friday as it flew just off the Mediterranean coast, Syria said. A Syrian military spokesman said anti-aircraft artillery shot down what was an unidentified aircraft that entered its airspace at a very low altitude and high speed.