Turkish artillery has renewed firing at targets in Syria after shells from across the border on Wednesday killed five Turkish nationals.
Several Syrian troops were killed by Turkish fire, activists from the Syrian Observatory for Human Rights said.
Turkey's border town of Akcakala was shelled, apparently by Syrian government forces, on Wednesday, killing a woman and three children. Homes and workplaces were damaged by mortar shells last Friday and in June Syria shot down a Turkish reconnaissance jet.
The United Nations Security Council is due to meet on Thursday to condemn Syria's actions, while Turkey's parliament is holding a special session for the government to seek approval for cross-border military operations, the BBC reports.
Ankara's response marks the first time it has fired into Syria during the 18-month-long unrest there. The country is sheltering more than 90,000 Syrian refugees.
Turkey also asked the UN Security Council to take "necessary action" to stop Syrian "aggression". The request was made by its envoy to the UN, Ertugrul Apakan, in a letter to the current president of the 15-member council, Guatemalan ambassador Gert Rosenthal.
In a statement on Wednesday, Prime Minister Tayyip Erdogan's office said:
"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.
"Turkey will never leave unanswered such kinds of provocation by the Syrian regime against our national security."
Meanwhile, NATO envoys held an urgent meeting in Brussels at the request of Turkey, who is a member of the military alliance.
The bloc issued a statement saying it "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 NATO ambassadors also expressed appreciation for Turkey's restraint in its response.