A woman and her three daughters were killed in Akcakale yesterday along with another woman.
Sanliurfa Province Governor Celalettin Guvenc said three or four Syrian shells fell in the border town.
Hundreds of residents marched to the local governor's office after the incident, demanding the resignation of town authorities, CNN-Turk said.
"Our armed forces units on the border immediately returned fire within rules of engagement, with artillery units hitting targets detected by radar inside Syria, Prime Minister Recep Tayyip Erdogan's office said in a statement. " Turkey will never let such provocations by the regime in Syria go unanswered."
Syrian leaders have blamed Turkey for aiding rebels who are fighting to oust President Bashar al-Assad. The Turkish government, which hosts about 94,000 Syrian refugees, has allowed some opposition fighters to use the country as a base.
US Secretary of State Hillary Clinton told reporters in Washington that "we are outraged" by the attack and called the situation in Syria "very, very dangerous."
Mrs Clinton said she plans to speak by phone with Turkish Foreign Minister Ahmet Davutoglu today.
Turkey's Deputy Prime Minister Bulent Arinc said that members of the North Atlantic Treaty Organisation have responsibilities in the "face of such an attack on a member country."
It wasn't clear what other steps Turkey may be planning.
Turkey's Foreign Ministry convened an emergency meeting over the incident. Mr Davutoglu briefed Lakhdar Brahimi, the United Nations and Arab League special envoy, about the matter, NTV said. He also called UN secretary-general Ban Ki-moon and NATO secretary-general Anders Fogh Rasmussen to discuss the incident, the ministry said.
NTV also reported that NATO was to convene "urgently" last night over the shelling in Turkey.
Meanwhile, a series of huge explosions tore through the regime-held heart of Syria's biggest city yesterday levelling major Aleppo buildings and killing scores of people, many of them soldiers.
At least two and possibly three explosions struck the main Saadallah Jabri Square, the seat of a number of important government buildings, along with two government-owned hotels, a military officers' club, and a telecoms office.
Syria's state news agency put the death toll at 34. The Syrian Observatory for Human Rights said 48 people were killed.
"This area was where the regime housed many of its officers, and the hotels were full of its soldiers," said a resident.
State media said the explosions were caused by suicide car bombers.
There were immediate, competing conspiracy theories about whether the government version of events was true. (© Daily Telegraph, London)