Turkey has renewed firing at targets inside Syria after two women and three children were killed by shelling from across the border on Wednesday.
Several Syrian troops were killed by Turkish fire, a UK-based Syrian activist group said.
Syria says it is investigating the shelling in the town of Akcakale.
The Turkish parliament is discussing authorising troops to cross into Syria. But government sources say Turkey is not planning to declare war on Syria.
A government official said the retaliatory shelling - now in its second day - was only a "warning" to the authorities in Damascus.
The UN Security Council is to meet later, following a Turkish request for the body to take "necessary action" to stop Syrian "aggression".
Nato has held an urgent meeting to support Turkey, demanding "the immediate cessation of such aggressive acts against an ally".
The US, the UK, France and the European Union have already condemned Syria's actions.
Russia, which is allied to President Bashar al-Assad's government, has asked Damascus to acknowledge officially that the cross-border attack was "a tragic accident" which will not happen again.
In Syria itself as many as 21 members of Syria's elite Republican Guards have been killed in an explosion and firefight in the Qudsaya district of Damascus, the UK-based Syrian Observatory for Human Rights told the BBC.
Turkish security officials said Turkey resumed artillery strikes on Syria early on Thursday, targeting the Tall al-Abyad district, some 10km (six 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.
Ankara's response marks the first time it has fired into Syria during the 18-month-long unrest there.
The office of Turkey's Prime Minister Recep Tayyip Erdogan said on Wednesday that Turkey would "never leave unanswered such kinds of provocation by the Syrian regime against our national security".
Parliament in Ankara is meeting in an emergency session in response to a government request for the military to be authorised to launch cross-border operations.
Akcakale has been fired on several times over the past few weeks.
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.
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.
Syria's Information Minister Omran Zoabi offered Damascus's "sincere condolences to the families of the victims and to our friends the Turkish people".
Without admitting the shelling, Mr Zoabi said: "The border with Turkey is long, illegal arms trafficking takes place along the border and armed groups move along the border."
Syria, he said, respected the sovereignty of neighbouring countries.
Wednesday's attack is believed to be only the second time that people have died as a result of violence spilling over the border from Syria into Turkey.
Two Syrian nationals were killed on Turkish soil in April by stray bullets fired from Syria.
The BBC's Jim Muir in Lebanon says Ankara is not interested in a confrontation and will keep its retaliation to a minimum.
The Turkish armed forces have in the past moved into northern Iraq in pursuit of Kurdish militants who had bases there.