Syria's conflict escalated when deadly cross-border shelling into Turkey drew return fire that continued after dark, as well as sharp condemnation from NATO and major Western powers.
Inside the conflict-torn country, 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 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.
Internationally, Damascus drew harsh condemnation from Turkey as well as NATO and Western powers for the cross-border shelling Wednesday afternoon that claimed five Turkish lives, among them a woman and three children.
Ten others were reported wounded, two of them critically.
A furious Turkey returned fire, with shells still raining down on Syria after nightfall, while Ankara called on the UN Security Council to take "necessary action" to halt Syrian aggression against its territory.
In Ankara, Prime Minister Recep Tayyip Erdogan's cabinet was set to seek clearance Thursday for possible future cross-border military operations "when deemed necessary", at a special parliament session.
After the shells hit the border town of Akcakale Erdogan's office said "our armed forces in the border region immediately retaliated against this heinous attack... by shelling targets spotted by radar.
"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.
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 behavior of the Syrian regime, and why it must go."
The incident was the most serious between the neighbours since June, when Syria shot down a Turkish fighter plane with the loss of its two-man crew and Ankara took the matter to NATO.
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 is battling foreign-backed "terrorists," whom it has frequently accused Ankara of training and financing.
Inside Syria, after the two blasts near the Aleppo officers' club, a third bomb exploded at an entrance to the Old City in Bab Jnein, the Syrian Observatory for Human Rights and a military official said.
"We heard two enormous explosions, as though the gates of hell were opening," said Hassan, an employee of a nearby hotel. "I saw thick smoke, and I helped a woman on the pavement whose arms and legs were completely dislocated."
The owner of a shop a block away said: "I pulled out from the rubble a child less than 10 years old who has lost a leg."
Rebels meanwhile attacked a political intelligence branch in Aleppo and a market where many troops were posted, the Observatory reported.
"The rebels are now attacking regime troops in the heart of the city," its director Abdel Rahman told AFP. "This is part of the decisive battle, and the regime can no longer claim to control the city."
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.
In the northwestern province of Idlib, rebels killed at least 15 troops when they attacked and destroyed three army posts in the village of Bdama, near Jisr al-Shughur, said Abdel Rahman.
Army shelling and helicopter gunfire killed at least 16 people including three children in Sahn, a village in the central province of Hama where rebels have a strong presence, he added.
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.