It has been a night of intense political activity in Turkey. It comes after the Turkish military struck at targets inside Syria, after a mortar bomb fired from inside Syria killed five people in Turkey.
Military compounds in Idlib came under fire after the earlier attack on the Turkish border town of Akcakale.
NATO has expressed solidarity with Ankara, an important member.
"There has been an attack on the Turkish mainland and Turkish citizens have lost their lives," said Deputy Prime Minister Bulent Arinc. "International law definitely allows for a response. Turkey is a member of NATO and certain NATO treaties entail responsibilities when a member nation is attacked."
Ankara has also asked the UN to take action. Security Council diplomats say a statement will be issued on Thursday.
The Turkish Parliament will meet this morning for an emergency session and may consider further military action.
Speaking on state television, Syria's information minister Omran Zoabi said the deaths are being investigated. He sent his condolences to the families of the victims and said the Turkish people would remain brothers and sisters.
Analysts say the attack takes the hitherto domestic conflict in Syria onto another level, as it involves the deaths of citizens in a neighbouring sovereign state.
