Turkey says its armed forces have fired on targets inside Syria in retaliation for a Syrian mortar attack that killed five Turkish civilians in a Turkish border town.

Turkish Prime Minister Recep Tayyip Erdogan's office said Turkish forces in the border region immediately shelled Syrian targets spotted by radar Wednesday, in response to what it called a “heinous” attack on the Turkish town of Akcakale.

Akcakale mayor Abdulhakim Ayhan told CNN Turk television that a Syrian mortar struck a house in the community earlier Wednesday. He said a woman and a child were among the dead.

Turkish Foreign Minister Ahmet Davutoglu protested the Syrian attack to the NATO alliance, of which Ankara is a member, and to the United Nations.

A NATO official told VOA that alliance ambassadors were meeting in emergency session Wednesday to discuss the situation.

U.N. Secretary-General Ban Ki-moon responded to the cross-border fighting by urging Syria to respect the territorial sovereignty of its neighbors. He said the escalation shows how the Syrian conflict is increasingly harming neighboring states.

U.S. Secretary of State Hillary Clinton said Washington is “outraged” at the Syrian mortar strike on Turkey, a fellow NATO alliance member. Speaking Wednesday, she expressed regret at the loss of Turkish lives and said she would speak to Davutoglu, her Turkish counterpart, to discuss next steps.