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 Akcakale was shelled, apparently by Syrian government forces, on Wednesday, killing a woman and three children.
The UN Security Council is due to meet later to condemn Syria’s actions.
Ankara’s response marks the first time it has fired into Syria during the 18-month-long unrest there.
Turkey also asked the UN Security Council to take “necessary action” to stop Syrian “aggression”.
Meanwhile, Nato envoys held an urgent meeting in Brussels at the request of Turkey, which 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 government in Ankara is expected to ask parliament shortly 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.
‘Abominable attack’
Turkish security officials said Turkey resumed artillery strikes on Syria early on Thursday, targeting the Tall al-Abyad district, some 10km (6 miles) inside the Syrian border.
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.”
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.
BBC
