Turkey stepped up retaliatory artillery strikes on a Syrian border town on Thursday, killing several Syrian soldiers, while its parliament debated authorising further military action in the event of another spillover of the Syrian conflict.
Syria's staunch ally Russia said it had received assurances from Damascus that the mortar strike had been a tragic accident and would not happen again and Syria's Information Minister conveyed his condolences to the Turkish people.
But Turkey's government said "aggressive action" against its territory by Syria's military had become a serious threat to its national security and sought parliamentary approval for the deployment of Turkish troops beyond its borders.
"Turkey has no interest in a war with Syria. But Turkey is capable of protecting its borders and will retaliate when necessary," İbrahim Kalın, a senior adviser to Prime Minister Tayyip Erdoğan, said on his Twitter account.
"Political, diplomatic initiatives will continue," he said.
In the most serious cross-border escalation of the 18-month uprising in Syria, Turkey hit back after what it called "the last straw" when the mortar hit Akcakale, killing a mother, her three children and a female relative.
The Syrian Observatory for Human Rights said several Syrian soldiers were killed in the Turkish bombardment of a military post near the Syrian town of Tel Abyad, a few miles across the frontier from Akçakale. It did not say how many soldiers died.
"We know that they have suffered losses," a Turkish security source told Reuters, without giving further details.
The observatory also reported clashes between Syrian rebels and the Syrian army at the military post, and said the rebels had killed 21 elite Republican Guards on Thursday in an ambush on an army minibus in a suburb northwest of Damascus.
Syrian President Bashar al-Assad used force of arms to try to crush a peaceful pro-democracy movement when it erupted in March 2011.
He now faces a full-scale armed revolt that has brought rebels into the suburbs of Damascus and shows signs of becoming a sectarian conflict that could destablise neighbouring states including Turkey, Iraq and Lebanon.
Turkey's parliament had already been due to vote on Thursday on extending a five-year-old authorisation for foreign military operations, an agreement originally intended to allow strikes on Kurdish militant bases in northern Iraq.
But the memorandum signed by Erdogan and sent to parliament overnight said that despite repeated warnings and diplomatic initiatives, the Syrian military had launched aggressive action against Turkish territory, presenting a "serious threat".
"At this point the need has emerged to take the necessary measures to act promptly and swiftly against additional risks and threats," it said.
Police fired tear gas to stop a group of around 25-30 anti-war protesters chanting "We don't want war!" and "The Syrian people are our brothers!" from approaching parliament as deputies debated the motion.
It was not clear who fired the mortar into Turkey, but security sources said it had come from near Tel Abyad and that Turkey was increasing the number of troops along its border. Syria said it was investigating the source of the mortar bomb and urged restraint. Information Minister Omran Zoabi said his country respected the sovereignty of neighbouring countries.
Russia said Damascus had vowed there would not be a repeat.
"We think it is of fundamental importance for Damascus to state that officially," RIA Novosti news agency quoted Russian Foreign Minister Sergei Lavrov as saying during a visit to Islamabad.
Some residents of Akcakale spent the night on the streets, while others gathered outside the mayor's office, afraid to return to their homes along the border as the dull thud of distant artillery fire rumbled across the town.
"We haven't been able to sleep in our own homes for 15 days, we had to sleep in our relatives' houses further away from the border because it's not safe down there," said shopkeeper Hadi Celik, 42, a father of five.
Turkey's military response contrasted with its relative restraint when Syria shot down a Turkish reconnaissance jet in June. Ankara increased its military presence along its 900-km (560-mile) border with Syria and called a meeting of NATO's North Atlantic Council.
At the time, Erdoğan warned any Syrian element approaching Turkey's border and deemed a threat would be treated as a military target.
World leaders condemned the mortar strike but urged restraint.
British Foreign Secretary William Hague told Reuters the Turkish response was "understandable" but warned against an escalation, while EU foreign policy chief Catherine Ashton called on Syria to respect the territorial integrity of its neighbours.
NATO said it stood by member-nation Turkey and urged Syria to put an end to "flagrant violations of international law."
The US-led Western military alliance held an urgent late night meeting in Brussels to discuss the matter and in New York, Turkey asked the UN Security Council to take the "necessary action" to stop Syrian aggression.
UN diplomats said Security Council members hoped it would issue a non-binding statement on Thursday that would condemn the mortar attack "in the strongest terms" and demand an end to violations of Turkey's territorial sovereignty.
Members had hoped to issue the statement on Wednesday, but Russia - a staunch ally of Syria's, which along with China has vetoed three UN resolutions condemning Assad's government -asked for a delay, diplomats said. France said the Security Council must send a swift and clear message.
Some 30,000 people have been killed across Syria, activists say, in a conflict with growing sectarian overtones which threatens to draw in regional Sunni Muslim and Shi'ite powers.
Turkey is sheltering more than 90,000 refugees from Syria and fears a mass influx similar to the flight of half a million Iraqi Kurds into Turkey after the 1991 Gulf War.
Violence inside Syria intensified on Wednesday with three suicide car bombs and a mortar barrage ripping through a government-controlled district of Aleppo housing a military officers' club, killing 48 people, according to activists.