Turkey has contacted the UN and Nato after Syrian shells killed five people in a Turkish town near the border between the two countries.
The shells exploded after being fired into Akcakale from Tall al-Abyad in Syria, where forces loyal to President Bashar al-Assad are trying to put down an 18-month-old insurgency.
The dead are said to include a woman and her three children.
Later, reports said Turkey had struck back at Syrian targets.
A statement from Prime Minister Recep Tayyip Erdogan said Turkish forces had shelled targets along the border identified by radar, AFP news agency reported.
Turkish Deputy Prime Minister Bulent Arinc was quoted as saying that Syria must be made to account for the incident and there must be a response under international law.
Not to get overly dramatic, but there are potentially the seeds of a major conflict in this. Turkey is a US ally and member of NATO. Syria is a Russian and Iranian ally, and the Russians and Iranians have been assisting Assad during the rebellion. Add to that, the chaotic nature of the war in Syria.