BEIRUT -- President Bashar Assad has exploited his greatest advantage in the Syrian civil war -- his air power -- to push back rebel advances and prevent the opposition from setting up a rival government in its northern stronghold.
Along the way, fighter jets and helicopters have hit civilian targets such as hospitals, bakeries and residential buildings, according to a report released Thursday by a U.S.-based human rights group. It accused the regime of committing war crimes with indiscriminate airstrikes that have killed more than 4,000 civilians since summer.
The Human Rights Watch report said Assad's air force has dropped "imprecise and inherently indiscriminate" munitions, including cluster bombs, on civilian areas.
In recent months, large parts of northern Syria near the border with Turkey have fallen to the rebels, including several neighbourhoods of Aleppo, the country's largest city. With the recent influx of more advanced weapons and other foreign aid, the rebels have also made major gains in the south, seizing military bases and towns in the strategically important region between Damascus and the border with Jordan.
Two years into the uprising, however, the Assad regime's control of the skies is hampering rebels' efforts to hold on to territory they capture with any efficiency. An interim leader of the opposition has been elected, but he and others opposed to Assad have made only a few, brief forays into rebel-controlled areas.
"The air force is extremely important for Assad right now," said Joseph Holliday, a Syria analyst at the Washington-based Institute for the Study of War.
"It has allowed Assad to prevent rebels from establishing a part of Syria where people can be safe and the opposition can focus on governing the place," he said. "It very difficult to do that without a space that is free from constant harassment from the aircraft."
Although the rebels have been able to shoot down several aircraft after capturing some heavier weapons from military bases, they are largely helpless when it comes to Assad's air supremacy.
The opposition has repeatedly asked their foreign backers for weapons that can shoot down the regime's aircraft and help hasten the fall of Assad. But the United States and its European allies have been reluctant to provide opposition fighters with anti-aircraft missiles for fear they may end in the hands of radical Muslim groups that have been the most organized fighting force on the opposition side.
The rebels also want a no-fly zone established over northern Syria, but the countries opposed to Assad have taken no action on that option, either.
The top U.S. military commander in Europe, Adm. James Stavridis, said last month that some NATO nations are looking at a variety of military operations to end the deadlock and assist the opposition forces, including using aircraft to impose a no-fly zone, providing military assistance to the rebels and imposing arms embargoes.
As with U.S. and international involvement in Libya in 2011, a resolution from the UN Security Council and agreement among the alliance's 28 members would be needed before NATO takes a military role in Syria, Stavridis said.
Late last year, NATO deployed Patriot batteries along Turkey's border with Syria, with the alliance's leaders emphasizing that the missiles will not be used to shoot down aircraft operating in Syrian airspace.
Military experts say it is unlikely the West will revisit the no-fly zone any time soon.
"It's not easy to just go on and establish a no-fly zone, and the West has said so before," said Beirut-based military analyst, Brig. Gen. Hisham Jaber, a retired Lebanese army officer who heads the Middle East Center for Studies and Political Research in Beirut.
"They know that the Syrian army remained strong, the air force is behind Assad, and they also know that Syria has a very sophisticated air defence system," he said.
Jaber said Syria has about 400 operational aircraft, although analysts say it is difficult to come up with reliable figures on the Syrian air force and air defences because of the extreme secrecy that blankets its military matters.
Like his late father and predecessor, Hafez, the younger Assad stacked key military posts with members of their minority Alawite sect in the past 40 years, ensuring the loyalty of the armed forces by melding the fate of the army and the regime. The air force is particularly close to the regime. Hafez Assad was an air force pilot and commander before seizing power in 1970.
However, many pilots are Sunni Muslims, and defections from the air force since the beginning of the uprising in March 2011 have been rare.
In the most dramatic escape from Syria, a fighter pilot on a training mission flew his MiG-21 warplane to neighbouring Jordan in June. Only three other pilots have reportedly defected, crossing into Jordan by land last summer.
Syrian officials denounced the pilot, who is a Sunni Muslim, and military experts with knowledge of Syria suggested that the regime has grounded Sunni pilots, relying on Alawites to carry out airstrikes instead. No order has appeared in public to support their claims.
While the air force is an important tool in Assad's battle for survival, it's not his last one, said Joshua Landis, a Syria expert at the University of Oklahoma.
"It's the 3 million Alawites who believe that they will be ethnically cleansed by the opposition if the rebels overthrow Assad," Landis said. "It's because of the fear of those who could come after him that has spread around Syrian minority communities in the past year of the revolution, that many -- including Sunnis -- continue fighting on the regime's side."
In their campaign against the opposition, the Syrians have been using helicopters, MiG jets and trainer aircraft to hit targets daily in the north, the east, the south and in rebel strongholds on the edges of the capital of Damascus.
"The aim of the airstrikes appears to be to terrorize civilians from the air, particularly in the opposition-controlled areas where they would otherwise be fairly safe from any effects of fighting," Ole Solvang of Human Rights Watch told The Associated Press.
More than 4,300 people have been killed since summer in such attacks that amount to "serious violations of international humanitarian law," and people who commit such breaches are "responsible for war crimes," the New York-based group said in the report, the most comprehensive study of Syrian air force operations in rebel-held areas since the beginning of the conflict. The UN estimates that more than 70,000 people have been killed in the civil war.
The Assad regime has significantly increased its use of air power since July, when the rebels captured territory in the north after an offensive, as well as last month after they seized their first provincial capital, Raqqa. The city is bombed almost daily.
Human Rights Watch said it inspected 52 sites in northern Syria and documented what it labeled 59 unlawful attacks by the Syrian air force in rebel-held areas.
Based on inspections and more than 140 interviews with witnesses, HRW said warplanes "deliberately targeted four bakeries (in the north) where civilians were waiting in bread lines a total of eight times."
Repeated aerial attacks on two hospitals that the group visited in the northern areas under opposition control "strongly suggest that the government also deliberately targeted these facilities," HRW said.
Assad's jets have dropped incendiary weapons on residential areas in the north that are designed to set fire to objects and people, the group said.
The planes have also hit civilian areas with makeshift bombs, made from hundreds of pounds (kilograms) of explosives stuffed into barrels. HRW even found unexploded naval mines on sites hit by airstrikes in northern Syria, Solvang said.
"They seem to be using pretty much everything they have to bomb places," he said.
Officials in Damascus could not immediately be reached for comment on the Human Rights Watch report. The Syrian government describes the rebels as foreign-backed terrorists set on destroying the country.
HRW also criticized the Free Syrian Army and other opposition groups for not taking measures to avoid deploying forces and headquarters in or near densely populated areas.
