Car bombs tore Wednesday through Syria's second city Aleppo, leaving dozens dead, as shells from the conflict crashed into neighbouring Turkey, killing five people and prompting Ankara to strike back.
Two blasts went off in quick succession 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.
A third exploded soon after at an entrance to the Old City in Bab Jnein, the Syrian Observatory for Human Rights and a military official said.
A man whose family owns a coffee shop overlooking the square described the sound of the blasts as "terrifying".
"I ran to my parents' room and found their faces covered in blood," said the man, identifying himself only as Omar. "Most of the people rescued from under the rubble of the hotel were soldiers."
The Observatory said at least 48 people were killed and almost 100 wounded, adding "most of them were regime troops". An official said 37 people died.
"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."
The bloodshed spilled into neighbouring Turkey when shells hit the border town of Akcakale.
"Five people, including a mother and her three children, were killed," said Abdulhakim Ayhan, mayor of Akcakale, where the shells exploded after being fired from Tall al-Abyad on the Syrian side.
Ankara said it hit back with shelling of its own, and that it called an urgent meeting of NATO.
"Our armed forces in the border region immediately retaliated against this heinous attack... by shelling the targets spotted by radar," said the office of Prime Minister Recep Tayyip Erdogan.
"Turkey will never leave unanswered such provocations by the Syrian regime targeting our national security, in line with engagement rules and international law," said the statement.
Foreign Minister Ahmet Davutoglu had contacted NATO chief Anders Fogh Rasmussen, and "it has been decided that the NATO council meet urgently," it added.
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US Secretary of State Hillary Clinton said Washington was "outraged" by the shelling into Turkey, while UN chief Ban Ki-moon urged Syria to respect the territorial sovereignty of its neighbours.
Wednesday's shelling was the second time fire from Syria has killed people inside Turkey since the uprising against President Bashar al-Assad's regime erupted in March 2011.
On the battle front, rebels attacked a political intelligence branch in Aleppo, as well as a market where a large number of 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," he added.
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.
Since Thursday, the fighting has become more intense, 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, with more than 31,000 people killed, according to activists.
On July 18, rebels carried out a massive bombing on a complex in Damascus, killing four security chiefs, including President Assad's brother-in-law and the defence minister.
Troops have since pushed the rebels to the outskirts of the capital, but they have lost control of several border crossings and are battling to retake Aleppo.
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.