Turkey's military struck targets inside Syria on Wednesday in response to a mortar bomb fired from Syrian territory which killed five Turkish civilians, Turkish Prime Minister Tayyip Erdogan's office said in a statement.
"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," the statement said.
"Turkey will never leave unanswered such kinds of provocation by the Syrian regime against our national security."
Foreign Minister Ahmet Davutoglu had spoken by telephone with UN Secretary General Ban Ki-moon and the foreign ministers of several UN Security Council member countries about the incident, the statement said.
Davutoglu had also agreed with NATO Secretary-General Anders Fogh Rasmussen on the need for an emergency meeting of NATO members, the statement said.
The mortar bomb fired from Syria landed in a residential district of the southeastern Turkish town of Akcakale earlier Wednesday, killing a woman and four children from the same family and wounding at least eight other people.
A cloud of dust and smoke rose up over low-rise buildings as residents ran to help the wounded. Others, infuriated by the increasing spillover of violence from Syria's civil war, took to the streets shouting protests against the local authorities.
In response to the incident, UN Secretary-General Ban Ki-moon urged the Syrian government to respect the territorial integrity of its neighbors and warned that the 18-month-long conflict in Syria is increasingly harming other countries in the region.
"The Secretary-General calls on the Syria Government to respect fully the territorial integrity of its neighbors as well as to end the violence against the Syrian people," Ban's press office said in a statement.
Davutoglu had signaled over the weekend that Turkey would take action if there was a repeat of a mortar strike which damaged homes and workplaces in Akcakale last Friday.
"It (latest mortar round) hit right in the middle of the neighborhood. The wife and four children from the same family died," Ahmet Emin Meshurgul, local head of the Turkish Red Crescent, told Reuters, adding he knew the victims personally.
"People here are anxious, because we got hit before. Security forces tried to convince people to empty the neighborhood near the border, but now we've been hit right in the middle of the town," he said.
A Reuters witness saw three police officers among the wounded being taken to hospital.
Erdogan long cultivated good relations with Assad but became a harsh critic after Syria's popular revolt began last year, accusing him of creating a "terrorist state." Erdogan has allowed Syrian rebels to organize on Turkish soil and pushed for a foreign-protected safe zone inside Syria.