Turkish artillery has fired on positions inside Syria after shells from Syria killed five people in a southern Turkish border town.
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.
The Turkish armed forces have in the past moved into northern Iraq in pursuit of Kurdish militants who had bases there.
Turkey’s territory has been hit by fire from Syria on several occasions since the uprising against Mr Assad began, but Wednesday’s incident was the most serious.
In a statement, the office of Turkey’s Prime Minister Recep Tayyip Erdogan 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,” the statement said.
Syria said it was looking into the origin of the cross-border shelling that hit Akcakale.
Information Minister Omran Zoabi added: “Syria offers its sincere condolences to the families of the victims and to our friends the Turkish people.”
Turkish Foreign Minister Ahmet Davutoglu contacted UN Secretary-General Ban Ki-moon, the UN’s Syria peace envoy Lakhdar Brahimi and Nato secretary general Anders Fogh Rasmussen after the incident.
Mr Ban urged Damascus to respect the territorial sovereignty of its neighbours, saying the cross-border incident “demonstrated how Syria’s conflict is threatening not only the security of the Syrian people but increasingly causing harm to its neighbours”.
Mr Rasmussen told Turkey’s foreign minister that he strongly condemned the incident, a Nato spokeswoman said, and continued to follow developments in the region “closely and with great concern”.
Mr Rasmussen has repeatedly said that Nato has no intention of intervening in Syria but stands ready to defend Turkey if necessary.
US Secretary of State Hillary Clinton said: “We are outraged that the Syrians have been shooting across their border… and regretful of the loss of life on the Turkish side.”
UK Deputy Prime Minister Nick Clegg, who is in Turkey on a trade and diplomatic visit, said: “We condemn all violence by the Syrian regime and demand that it avoids any repetition of today’s incident on the border with Turkey.”
Akcakale has been fired on several times over the past few weeks.
The BBC’s Jim Muir says Syrian government forces are attempting to cut rebel supply routes by winning back the border crossing at Tall al-Abyad which the rebels seized last month.
Residents have been advised to stay away from the border, and more than 100 schools have been closed in the region because of the violence in neighbouring Syria.
Turkey’s state-owned Anatolia news agency reported that angry townspeople had marched to the mayor’s office to protest about the deaths on Wednesday.
Town mayor Abdulhakim Ayhan said: “There is anger in our community against Syria,” adding that stray bullets and shells had panicked residents over the past 10 days.
Wednesday’s attack is believed to be only the second time that people have died as a result of violence spilling over the border from Syria into Turkey.
Two Syrian nationals were killed on Turkish soil in April by stray bullets fired from Syria.
In Syria itself, at least 34 people were killed and dozens wounded in a series of bomb explosions in the centre of Syria’s second city, Aleppo, on Wednesday.
The attacks levelled buildings in the city’s main square. A military officers’ club and a hotel being used by the military bore the brunt of the blasts, some of which were carried out by suicide car bombers.