Turkey resumed pounding Syrian targets early on Thursday, reportedly killing several soldiers, in reprisal for deadly cross-border fire that has sharply escalated tensions in the tinder-box region.
"Artillery fire resumed at 0300 GMT this morning," a Turkish security source told AFP on condition of anonymity.
Several Syrian soldiers were killed in overnight Turkish shelling across the border, the Syrian Observatory for Human Rights watchdog said earlier, without giving an exact figure.
The escalation came after mortar fire from Syria crashed inside the Turkish border town of Akcakale on Wednesday, killing five civilians, including a mother and her three children.
It marked the first time that Turkish citizens had been killed as a result of fire from Syria since the uprising against President Bashar al-Assad's regime began in March 2011.
Turkey has demanded that the UN Security Council take action against Damascus after Wednesday's attack, which drew sharp Western condemnation, with the US calling it 'depraved.'
Later Thursday the Turkish parliament was to convene a special session to consider a request from Prime Minister Recep Tayyip Erdogan's cabinet for approval of possible future cross-border military operations "when deemed necessary."
Wednesday's shelling marked the most serious incident between onetime allies Syria and Turkey since June, when Syria shot down a Turkish fighter jet that had briefly strayed into its airspace, killing the two crewmen.
A furious Ankara returned fire, called an emergency meeting of the North Atlantic Treaty Organisation (NATO)and demanded action from the UN Security Council.
"Our armed forces in the border region immediately retaliated against this heinous attack... by shelling targets spotted by radar," Erdogan's office said in a statement.
"Turkey will never leave unanswered such provocations by the Syrian regime targeting our national security, in line with engagement rules and international law," his office said.
And in a letter to the UN Security Council, Ankara said: "This is an act of aggression by Syria against Turkey.
"It constitutes a flagrant violation of international law as well as a breach of international peace and security. Turkey calls for an immediate end to such unacceptable violations."
NATO, in an emergency meeting, said that it stood by its member Turkey and "urges the Syrian regime to end flagrant violations of international law."
Syria's "recent aggressive acts at NATO's southeastern border" were a "flagrant breach of international law and a clear and present danger" to an ally, it said.
US Secretary of State Hillary Clinton said Washington was "outraged", and Pentagon spokesman George Little said: "This is yet another example of the depraved behaviour of the Syrian regime, and why it must go."
Syrian Information Minister Omran Zoabi said in a statement reported by state television that Damascus was looking into the origin of the cross-border shelling.
"Syria offers it sincere condolences to the families of the victims and to our friends the Turkish people," he added.
The minister reiterated his government's charges that it was battling foreign-backed "terrorists" whom it has frequently accused Ankara of training and financing.
Inside Syria, meanwhile, bloody fighting raged between regime and rebel forces, and a series of massive bomb blasts ripped through the battleground of Aleppo, claiming scores of lives.
Two powerful blasts went off on Wednesday near a military officers' club around Aleppo's Saadallah al-Jabiri Square, ripping off a hotel's facade and flattening a two-storey cafe, an AFP correspondent reported.
The Syrian Observatory for Human Rights said at least 48 people were killed and almost 100 wounded in three separate blasts, adding "most of them were regime troops." An official said 37 people died.
Aleppo, with a population of 1.7 million people, has been one of the focal points of the conflict since mid-July, when the army promised the "mother of all battles" to clear the city of rebels.
The fighting has intensified in recent days, spreading at the weekend into the centuries-old, UNESCO-listed souk in the historic heart of Aleppo and sparking a fire that damaged hundreds of shops.
Bombings have increasingly become part of the unrest ravaging Syria, which began in March 2011 as peaceful protests for reform but has since escalated into an armed insurgency. More than 31,000 people have been killed since then, according to activists.
Nationwide, violence killed 147 people on Wednesday, said the Observatory.
UN-Arab League peace envoy Lakhdar Brahimi is due back in the region this week to try to revive talks aimed at ending the bloodshed, although the UN says it is still unclear if he will be able to enter Syria.