Turkey's military struck targets inside Syria on Wednesday in response to a mortar bomb fired from Syrian territory which killed five Turkish civilians, Prime Minister Tayyip Erdogan's office said in a statement.
"Our armed forces in the border region responded immediately to this abominable attack in line with their rules of engagement; targets were struck through artillery fire against places in Syria identified by radar," the statement said.
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"Turkey will never leave unanswered such kinds of provocation by the Syrian regime against our national security."
Late Wednesday the Turkish army continued shelling Syrian army positions along the border between the two countries, the Anatolia news agency reported.
The Turkish attacks, launched by a unit based in the city of Akcakale, near the border, caused heavy explosions on the Syrian side, according to the report.
Foreign Minister Ahmet Davutoglu had spoken by telephone with UN Secretary General Ban Ki-moon and the foreign ministers of several UN Security Council member countries about the incident, the statement said.
NATO demanded an immediate halt to "aggressive acts" against alliance member Turkey.
The shelling "constitutes a cause of greatest concern for, and is strongly condemned by, all allies", NATO ambassadors said in a statement, after they held a rare late-night meeting on Wednesday at Turkey's request to discuss the incident.
"The alliance continues to stand by Turkey and demands the immediate cessation of such aggressive acts against an ally, and urges the Syrian regime to put an end to flagrant violations of international law," the statement said.
On Wednesday night, Turkey asked the UN Security Council to take the "necessary action" to stop Syrian aggression and ensure that Syria respect its territorial sovereignty after a mortar bomb fired from Syria killed five Turkish civilians.
"This is an act of aggression by Syria against Turkey," Turkish UN Ambassador Ertugrul Apakan said in a letter to the president of the 15-nation Security Council, Guatemalan Ambassador Gert Rosenthal.
"It constitutes a flagrant violation of international law as well as a breach of international peace and security."
The Obama administration said it is "outraged" by the Syrian mortar shell that landed on a home in Turkey, killing at least five people.
Secretary of State Hillary Rodham Clinton said the US is consulting its NATO ally on what she termed a "very dangerous situation."
She plans to speak to Turkish Foreign Minister Ahmet Davutoglu later Wednesday.
Among the dead was a 6-year-old boy. Turkish state media reported angry townspeople marching to the local mayor's office to protest the deaths.
Speaking in Washington, Clinton said Syria's leaders were causing untold suffering to their own people, driven solely by their desire to cling to power.
She said responsible nations worldwide must band together to pressure Syria's government into a cease-fire and a political transition.
AP, Reuters, AFP contributed to the report