Turkey has hit targets inside Syria. Syria has hit targets inside Turkey. NATO is upset.
A woman and her three children were among those killed earlier when the shells, apparently fired by Syrian government forces, hit Akcakale.
Turkey's response marks the first time it has fired into Syria during the 18-month-long uprising against Syrian President Bashar al-Assad.
Nato ambassadors discussed the crisis.
After a meeting in Brussels, the military alliance issued a statement saying it "continues to stand by (Nato member) 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, the BBC's defence correspondent Caroline Wyatt reports.
At the same time, the government in Ankara is expected to ask the parliament on Thursday to authorise cross-border military operations in Syria, Turkish media report.
Separately, Bulent Arinc, deputy prime minister, said: "There has been an attack on Turkey's mainland and its citizens lost their lives. There is definitely a response to it in international law ... We are not blinded by rage but we will protect our rights to the end in the face of such an attack on our soil that killed our people."
Turkey has also asked the United Nations Security Council to take "necessary action" to stop Syrian aggression, calling the mortar attack "a flagrant violation of international law."Al Jazeera's Andrew Simmons, reporting from Antakya on the Turkish-Syrian border, said Arinc's mention of "certain responsibilities" contained within NATO treaty articles, could mean that Turkey responded without consulting international bodies first.
The Middle East is in turmoil... again? Still? Whatever. Things are getting rough. As a result, I have a question I didn't think I'd ask in this presidential election: Is foreign policy going to be an issue here? Originally, there wasn't a single reason to believe that foreign policy would make a difference, but now... I don't know.
See this post and others of mine here. Follow me on Twitter: @joec_esquire