Activists say Syrian forces are shelling several neighbourhoods in Aleppo as the battle for control of Syria's largest city rages into its sixth day.
Most fighters believe the bombardments will be followed by a ground campaign to pacify the city, although by Thursday morning reports of massive regime reinforcements had yet to materialise.
There were also reports that government forces were shelling Damascus, Idlib and Homs on Thursday.
A rebel spokesperson told the AFP news agency via Skype, however, that a "large number" of troops have been moved from the northwestern province of Idlib to Aleppo.
Last week, Syrian troops used a similar combination of aerial bombardments and overwhelming ground force to quash a rebel assault on Damascus.
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Local activist Mohammed Saeed told the AP news agency that the shelling seems to be random and clashes had spread to key neighbourhoods in the centre of the city.
The Britain-based Syrian Observatory for Human Rights said the fighting and shelling in Aleppo killed 26 people on Wednesday, including many children.
Herve Ladsous, the UN peacekeeping chief, said he had told Syrian officials that without a significant reduction in violence, the remaining 150 UN observers would leave on the expiry of the "final" 30-day extension of the mission's mandate, agreed by the Security Council on July 20.
Prime Minister Tayyip Erdogan said on Thursday Turkey could act against a "terrorist" organisation in northern Syria if it sees it as a threat, referring to outlawed Kurdish militants.
Erdogan's comments Thursday follow reports that Kurdish rebels and the Democratic Union Party of Syria took control of five cities along the Syrian-Turkish border.
"We will not allow a terrorist group to establish camps in northern Syria and threaten Turkey," Erdogan told a news
"If there is a step which needs to be taken against the terrorist group, we will definitely take this step," he said.
As the violence increases, high-level defections from Assad's regime are growing.
The US on Wednesday confirmed the defections of two senior Syrian diplomats: Abdel Latif Dabbagh, the ambassador to the UAE, and and his wife Lamia al-Hariri, the Syrian charge d'affaires in Cyprus.
White House spokesperson Jay Carney told reporters aboard Air Force One that the moves showed that "senior officials around the Assad inner circle are fleeing the government because of the heinous actions taken by Assad against his own people, and the recognition that Assad's days are numbered".
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Earlier, a senior state department official said: "These defections serve as a reminder that the bottom is starting to fall out of the regime. It is crumbling and losing its grip on power."
The Syrian foreign ministry downplayed the latest defection reports on Syrian state television on Thursday.
Lamia al-Hariri is not an ambassador, they said, and had an administrative rather than a diplomatic post in Cyprus.
Ban Ki-moon, the UN chief, in Bosnia to visit the site of the Srebrenica genocide in 1995, told the parliament there that "the international community is being tested in Syria".
Syrians continue to flee
The stream of refugees fleeing Syria has continued as more than 150 Syrian families crossed into Iraq on Wednesday.
The Iraqi government has set up two camps for the refugees, and on Tuesday, it opened its border crossings to allow Syrians escaping the violence to enter the country.
Turkey indefinitely closed three border crossings to Turkish nationals trying to get into Syria, citing security concerns.
The UN refugee agency in Geneva said about 300 people fled from Syria into Turkey on Tuesday night.
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Thousands of Syrians have taken shelter into neighbouring Lebanon and Turkey.
The Britain-based Syrian Observatory also reported clashes in the Hajar al-Aswad district of Damascus, one of the last remaining rebel bastions after 10 days of fighting in the capital.
In Hama province in central Syria, a couple and their two children were killed as they tried to flee shelling. A video distributed by the Observatory showed grisly footage of the bodies.
Nationwide, the monitoring group put the death toll at 108 by Wednesday evening - 57 civilians, 36 soldiers and 15 rebels, while it said 158 people were killed across Syria on Tuesday.
The rights group Amnesty International warned about disturbing reports of "summary executions" by both Syrian troops and rebels, calling them "serious violations of international law".