President Bashar Assad is still in the Syrian capital Damascus and retains the loyalty of his armed forces in the face of an advancing rebellion, the Israeli military said on Sunday as fighting in Syria continues to escalate.

"The [Syrian] military is still loyal to Assad, despite a very big wave of defections, and he and his family are still in Damascus," Brigadier-General Yoav Mordechai, chief spokesman for the IDF, said in an Israeli television interview.

There have been doubts about Assad's location since a bomb on Wednesday killed four members of his high command in Damascus. Assad has not spoken in public since and state TV has shown only footage of him swearing in a new defense minister.

Syrian forces bombarded parts of Damascus with helicopter gunships on Sunday, witnesses said, rounding people up as troops clawed back territory a week after opposition fighters launched a major assault on the capital.

In a further escalation of a conflict that opponents of Assad have turned into all-out civil war, fighting raged around the intelligence headquarters in the 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."

The rebels said they lost 12 men and 40 were wounded during a 24-day siege they mounted to take the Bab al-Salam border post. The rebel commanding officer in the area, Abu Omar, said they took the crossing point on Sunday without a fight.

"There were two armored vehicles at the gate and they escaped. This is a safe zone for us and we don't expect the army to come and attack," said Omar. "Eighty percent of the area here is in rebel hands anyway," he added.

The bombardments in Damascus and Deir al-Zor were some of the fiercest yet and showed Assad's determination to avenge a bomb on Wednesday that killed four members of his high command.

It was the gravest blow in a 16-month-old uprising that has turned into an armed revolt against four decades of Assad rule.

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.

Three people were killed and 50 others, mostly civilians, were wounded in the early morning bombardment, 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."

The neighborhood of Barzeh, one of three northern areas hit by helicopter fire, was also under siege, by troops from the elite fourth division.

The division is run by Assad's younger brother, Maher al-Assad, 41, who is widely seen as the muscle maintaining the Assad family's Alawite minority rule.

His 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 a "Damascus volcano" 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. Diplomats and opposition sources said government forces were focusing on strategic centers, with one Western diplomat comparing Assad to a doctor "abandoning the patient's limbs to save the organs".

On the Iraqi-Syrian border, Iraqi security and border officials said Syrian forces had reasserted control over the Yarubiya crossing point on the Syrian side of the frontier, briefly seized by rebels on Saturday.

Syrian opposition activists said several towns in Syria's Kurdish northeast had passed without a fight into local hands in recent days as central authority eroded.

The surge in violence has trapped millions of Syrians, turned sections of Damascus into ghost areas, and sent tens of thousands of refugees fleeing to neighboring Lebanon.

The UN Security Council has approved a 30-day extension for a ceasefire observer mission, but Secretary General Ban Ki-moon has recommended it focus on pursuing prospects for a political solution - effectively accepting there is no truce.

Regional and Western powers have voiced concern the conflict might become a full-blown sectarian war that could spill across borders. But Assad's opponents remain outgunned and divided.