Syrian troops have driven rebel fighters out of two districts of Damascus a week after the insurgents launched a major assault on the capital.
Government troops retook control of the Damascus neighborhood of Mezzeh on Sunday and executed at least 20 unarmed men who they suspected of aiding rebels, opposition activists in the district said.
In Barzeh, members of the Syrian army's Fourth Division under the command of President Bashar al-Assad's brother executed several young men during an operation to regain control of the northern Damascus district, a witness and activists said.
Government forces have launched a determined counter-offensive since rebels brought their battle to overthrow Assad to the capital and killed four of the president's closest associates in a bomb attack last Wednesday.
In a further escalation of a conflict rapidly becoming a civil war, fighting raged around the intelligence headquarters in Syria's biggest city, Aleppo, and in Deir al-Zor in the east.
Syrian forces regained control of one of two border crossings seized by rebels on the frontier with Iraq, Iraqi officials said, but rebels said they had captured a third border crossing with Turkey: Bab al-Salam, north of Aleppo.
"Seizing the border crossings does not have strategic importance but it has a psychological impact because it demoralizes Assad's force," a senior Syrian army defector in Turkey, Staff Brigadier Faiz Amr, told Reuters by phone.
"It's a show of progress for the revolutionaries, despite the superior firepower of Assad's troops."
Rebels also seized an army infantry school in the town of Musalmiyeh, 16 km (10 miles) north of Aleppo, and captured several loyalist officers, while others defected, a senior military defector in Turkey and rebel sources inside Syria said.
"This is of big strategic and symbolic importance. The school has ammunition depots and armored formations and it protects the northern gate to Aleppo," Brigadier General Mustafa al-Sheikh told Reuters by phone from the town of Apayden on the Turkish border.
The bombardments in Damascus and Deir al-Zor were some of the fiercest yet and showed Assad's determination to avenge the bomb attack, the most spectacular blow in a 16-month-old uprising against four decades of rule by the Assad family.
Rebels were driven from Mezzeh, the diplomatic district of Damascus, residents and opposition activists said, and more than 1,000 government troops and allied militiamen poured into the area, backed by armored vehicles, tanks and bulldozers.
Government forces executed at least 20 men, aged approximately 20 to 30, several activists said by phone from Mezzeh.
"Most had bullet holes, one with as many as 18. Three had their hands tied behind their back. Some of the men were in their pajamas. Several had their legs broken or fingers missing. Others were stabbed with knifes," said Bashir al-Kheir, one of the activists.
The early morning bombardment of the neighborhood killed three people and 50 others, mostly civilians, were wounded, said Thabet, a Mezzeh resident. "The district is besieged and the wounded are without medical care," he said.
"I saw men stripped to their underwear. Three buses took detainees from al-Farouk, including women and whole families. Several houses have been set on fire."
Opposition and rebel sources say the guerrilla fighters in the capital may lack the supply lines to remain there for long and may have to make tactical withdrawals.
The neighborhood of Barzeh, one of three northern areas hit by helicopter fire, was overrun by troops commanded by President Assad's brother, Maher al-Assad, 41, who is widely seen as the muscle maintaining the Assad family's Alawite minority rule.
"At least 20 Fourth Division tanks and hundreds of its members entered Barzeh this afternoon," opposition activist Abu Kais said by phone from the district.
"I saw troops go into the home of 26-year-old Issa al-Arab. They left him dead with two bullets in his head."
He said people sheltering from the fighting had told him of the summary execution of a 17-year-old, Issa Wahbeh, who was pulled from the shelter and beaten and killed.
Mazen, another opposition activist in Barzeh, said the bodies had been found of four young men who appeared to have been shot at point blank range.
Syrian state television quoted a media source denying that helicopters had fired on the capital. "The situation in Damascus is normal, but the security forces are pursuing the remnants of the terrorists in some streets," it said.
Maher's role has become more crucial since Assad's defense and intelligence ministers, a top general and his powerful brother-in-law were killed by the bomb on Wednesday, part of an assault by rebels seeking to turn the tables in a revolt inspired by Arab Spring uprisings in Tunisia, Libya and Egypt.
Assad has not spoken in public since the bombing, but the Israeli military said it believed he was still in Damascus and retained the loyalty of his armed forces.
The Syrian Observatory for Human Rights said 1,261 people had been killed across Syria since last Sunday, when the fighting escalated in Damascus, including 299 of Assad's forces.
This made it by far the bloodiest week in an uprising that has claimed the lives of 18,000 people. A total of 79 civilians and 24 soldiers were killed on Sunday, the observatory said.
Most shops in Damascus were closed and there was only light traffic - although more than in the past few days. Some police checkpoints, abandoned earlier in the week, were manned again.
Many petrol stations were closed, having run out of fuel, and those that were open had huge lines of cars waiting to fill up. Residents reported long queues at bakeries.
Elsewhere, Iraqi officials said Syrian forces had regained control of the Syrian side of the Yarubiya border crossing, briefly seized by rebels on Saturday.
Iraq has said it cannot help Syrians fleeing the violence, and the border was sealed by the Iraqi army on Friday.
Regional and Western powers fear the conflict might become a full-blown sectarian war that could spill across borders, but have yet to find a coherent strategy to prevent this.
Arab League ministers meeting in Doha urged the opposition and the rebel Free Syrian Army to form a transitional government, Qatari Prime Minister Sheikh Hamad bin Jassim al-Thani told a news conference in Doha.
He said Arab countries would help to ensure safe passage out of Syria for President Assad if he stepped down quickly - something he has shown no inclination to do.
(Additional reporting by Oliver Holmes in Beirut, Fadhil Al-Badran in Falluja, Iraq, Suleiman al-Khalidi in Hacipasa, Turkey, Igor Ilic in Brijuni, Croatia, Leigh Thomas in Paris, Jamal al-Badrani in Mosul, Iraq, Sylvia Westall in Baghdad and Jonathan Burch in Cilvegozu, Turkey; Writing by Philippa Fletcher; Editing by Giles Elgood, Alessandra Rizzo and Eric Beech)