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 responded “immediately” by using radar to shell the sources of the Syrian mortar fire.

Mr. Erdogan said Ankara acted within international law and will never fail to retaliate for what he called Syrian provocations against Turkey's national security.

Turkish authorities said a woman and her three children were among those killed when Syrian mortars struck a residential area in Akcakale on Wednesday. They said at least 10 other people were wounded.

Syrian Information Minister Omran Zoabi said Damascus is investigating the origin of the mortar fire. He said Syria offers its condolences to the Turkish people for the deaths in Akcakale.

Zoabi also called on neighboring states to control their borders with Syria to prevent “terrorists” from crossing into Syrian territory. Syria uses the term terrorists to describe rebels fighting an 18-month uprising against autocratic Syrian President Bashar al-Assad.

U.S. Secretary of State Hillary Clinton said Washington is “outraged” at the Syrian mortar strike on Turkey, a fellow member of the NATO alliance.

The U.S. Defense Department said the attack is another example of what it called the “depraved behavior of the Syrian regime.” It said the United States stands by Turkey as a “strong ally.”

NATO ambassadors held an urgent meeting in Brussels on Wednesday at Turkey's request, and issued a statement demanding the “immediate cessation of such aggressive acts against an ally.”

NATO also urged the Syrian government to “end flagrant violations of international law.”

U.N. Secretary-General Ban Ki-moon called on Syria to respect the territorial sovereignty of its neighbors. He said the escalation shows how the Syrian conflict is increasingly harming neighboring states.

Mr. Ban's office said the U.N. chief discussed the situation with Turkish Foreign Minister Ahmet Davutoglu.

It said Mr. Ban “encouraged the minister to keep open all channels of communications with Syrian authorities with a view to lessening any tension that could build up as a result of the (Akcakale) incident.”

U.N. diplomats said the U.N. Security Council was discussing a possible statement in reaction to the cross-border exchange of fire.