Two bombs exploded simultaneously next to Syrian army compounds in the northern city of Aleppo, killing and wounding scores of President Bashar al-Assad's forces, residents and opposition activists said.
The bombs targeted makeshift barracks and the military police headquarters, situated in two adjacent sealed off districts in the centre of the city, said several residents and opposition campaigners from Aleppo.
The state news agency said an explosion near a hospital and a school in the Municipal Stadium district on Sunday killed 17 people and wounded at least 40. Residents said the facilities were used to house soldiers fighting an 18-month uprising against Assad.
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"The army had taken over the neighbourhood and emptied it from residents. The hospital was turned into army barracks," said activist Ahmad Saeed.
A woman living near the area said the casualty figure "appeared to be over 100", judging from the number of ambulances ferrying the wounded and dead from the area.
The Noble Aleppo brigade of the Free Syrian Army said in a statement it carried out the Municipal Stadium district attack, killing or wounding 200 troops. It said the bombs were planted inside the buildings in co-operation with a loyalist sympathiser.
Syrian warplanes earlier bombed a residential district to the east and exacerbated a water shortage in Syria's biggest city after a pipeline burst, activists said.
Sunday's air raid destroyed a residential complex in the Hananu neighbourhood, one of several in eastern Aleppo under rebel control, opposition activists told Reuters news agency by phone.
The death toll was at least five, including one woman, and bodies and wounded people were being dug out from the rubble. Video footage from the area showed scores of people searching and digging in the debris of a flattened building.
Syrian state media said four people were killed in a "terrorist attack" that targeted a bus in the province of Homs. The London-based Syrian Observatory for Human Rights said civilians and soldiers were in the bus.
Assad, whose family has ruled Syria for 42 years, has repeatedly said the revolt is the handiwork of Islamist "terrorists" and not a popular movement for democratic change.
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Hillary Clinton, US secretary of state, said on Sunday she was pessimistic about closing the gap with Russia on how to defuse the Syrian conflict before world leaders gather for the UN General Assembly later this month.
Clinton said she made the case for increasing pressure on Assad in talks with Russian President Vladimir Putin and Foreign Minister Sergei Lavrov at a summit of Pacific Rim states in Vladivostok, Russia over the weekend.
Chinese and Russian leaders restated their firm opposition to what they see as US meddling in Syria.
"Our US partners prefer measures like threats, increased pressure and new sanctions against both Syria and Iran. We do not agree with this in principle," Lavrov said.
Turkey, Saudi Arabia and most Arab nations have sided with their Syrian Sunni Muslim co-religionists at the forefront of the revolt.