BEIRUT (AP) — Its forces stretched thin on multiple fronts, President Bashar Assad’s regime has significantly increased its use of air power against Syrian rebels in recent weeks, causing a spike in civilian casualties.

The shift is providing useful clues about the capability of the Syrian air force as Western powers consider the option of enforcing a no-fly zone over the northern part of the country, where rebels control large swaths of territory along the Turkish border.

If a no-fly zone is enforced, Western aircraft will likely go head-to-head with the Syrian air force as well as try to neutralize, at least partially, its air defense system.

“It is certainly a sign of increased concern on the part of the regime,” said senior political scientist Christopher S. Chivvis of the Rand Corporation. “The growing use of air power, particularly fixed-wing aircraft, raises the possibility of foreign intervention.”

U.S. Defense Secretary Leon Panetta said Aug. 14 that plans to set up a no-fly zone over parts of Syria is “not on the front burner,” despite persistent calls from rebel forces there that they need the added protection from escalating regime airstrikes in the civil war.

Secretary of State Hillary Rodham Clinton has said that the U.S. and Turkey are discussing a range of steps, including a no-fly zone over some parts of Syria. Russia, Assad’s main foreign backer, opposes a no-fly zone.

There are no estimates on how many civilians have been killed in airstrikes in the four-week period in which stepped up attacks have been detected, but Syrian activists speak of hundreds, maybe thousands, across the country.

Helpless civilians run for cover when they hear the distant buzz of aircraft. Terrified and screaming, they head to the nearest basement or ground floor.

“May God exact retribution on him!” they shout against Assad or the pilot involved. “May God cripple him!” and “May he rot in hell!” are other frequently used curses.

Syria’s air force has an unimpressive record — it lost 85 planes in a battle with Israel in 1982 — but without any effective anti-aircraft weapons in the hands of the rebels, the relatively antiquated fleet of Soviet-era warplanes is unchallenged.

“We used to think mortars are bad,” activist El-Saeed Mohammed said by Skype from a Damascus suburb. “Now, we think they are almost sweet when compared to what warplanes do to us. There is no kidding with planes.”

He recounted how two elderly women died of heart attacks, possibly triggered by anxiety, during air raids in the past week in the greater Damascus area.

Besides the deadly airstrikes, activists and residents say, pilots also employ psychological tactics. Jets frequently swoop down in mock strafing raids, break the sound barrier over urban centers, or fly menacingly at high altitudes, a sign of a possible bombing raid.

Low flying helicopter gunships also spray streets with machine-gun fire, and residents lose sleep over air activity at night that frays the nerves of civilians and rebels alike.

The airstrikes often leave Syrians puzzled about what was the intended target.

On Monday, fighter jets bombed the town of Tel Rifat, about 20 miles (30 kilometers) north of the city of Aleppo, punching a six-foot-deep crater in the courtyard of a girls’ school and leveling five homes nearby, according to an Associated Press reporter who visited the scene.

No one was killed or injured, residents said, because the school and the homes were empty. Most of the town’s 35,000 people have fled due to frequent airstrikes and shelling by the government. The Brigade of Unification, the largest rebel group fighting in Aleppo province, has its headquarters in the town, but it has never been directly targeted.

It is not clear why the regime has decided to dramatically step up the use of air power, particularly fixed-wing warplanes, in a civil war fought mostly on the streets and with no clearly defined battle lines.

The rebels say it is a sign of the regime’s desperation after its ground forces began to lose their grip on the country. It is also likely the enhanced use of air power is designed to add firepower to an army stretched thin.

The Syrian army is fighting major battles in the greater Damascus area and the northern city of Aleppo, along with a multitude of fronts in the southern, central and eastern parts of the country.

Analysts say it is difficult to come up with reliable figures on the Syrian air force and air defenses because of the extreme secrecy that blankets its military matters. Some of the aircraft used so far also raise questions whether the regime — anticipating major battles against Western powers — is engaging in deception.

The Syrians have been using trainer aircraft fitted to operate as light ground support when they could have used some of the relatively modern Soviet-made MiG-25s and MiG-29s it is believed to have.

“So far, Syria has mainly used low-end systems, such as jet trainers or helicopter gunships, against the rebels,” said Pieter Wezeman, senior researcher at the Stockholm International Peace Research Institute.

“That raises the question why they have not used more advanced equipment,” he said. “Are they holding back, saving them for possible outside intervention?”

Chivvis, the Rand analyst, believes it could also be a question of the regime unable to trust pilots not to defect with relatively advanced aircraft that would be needed to resist later.

In June, a Syrian fighter pilot flew his MiG-21 warplane to neighboring Jordan, where he was given asylum.

Wezeman, who tracks Damascus’ arms purchases, said its strategy on air force and air defenses has changed since Israel’s 2007 airstrike against a suspected nuclear facility in eastern Syria.

Since then, he said, the regime has gone on a shopping spree for modern Russian-built aircraft and anti-aircraft missiles. Chivvis added that it may have spent more than $3 billion.

Purchases have included 24 late-model MiG-29M2 fighters and 36 Yak-130 combat training jets that can also carry modern weapons for ground attack missions, he said. Although Moscow has not delivered the MiGs and Yaks, SIPRI’s reports suggest that Russia continues to ship air defense missiles and other munitions to Syria.

In 2008, Russia’s ally Belarus delivered 33 second-hand MiG-23 fighter-bombers. It remains unclear whether these were used to bolster the air force’s numerical strength or to be used as parts for existing Mig-23s, he said.

SIPRI reported in 2009 that Syrian Mi-24 helicopter gunships had arrived in Russia for overhaul. But a shipment of Mi-24s back to Syria earlier this year was aborted after the British insurers of the vessel carrying them withdrew coverage.

In recent years, Syria has upgraded its extensive network of anti-aircraft missile batteries, purchasing new models and modernizing existing ones, including Russian-made 36 Pantsyr mobile surface-to-air missile systems and at least two of the eight Buk-M2E mobile SAMs on order. The Pantsyrs, considered particularly effective against attacking aircraft, feature a combination of 30mm cannons paired with a radar and anti-aircraft missiles all on the same vehicle.

Experts say that Syria possesses one of the most robust air defense networks in the region, with multiple SAMs providing overlapping coverage of key areas in combination with thousands of anti-aircraft guns capable of engaging attacking aircraft at lower levels.

Wezeman said that due to the secrecy involved, it was not possible to confirm whether the Syrians also obtained the advanced, Russian-built S-300 anti-aircraft missiles considered to be the cutting edge in aircraft interception technology.

Moscow had refused to deliver the systems, but there have been unconfirmed reports that other nations may have sent Syria the missiles, which could make any aerial intervention very costly for the attackers.

Separately, Syria also has obtained from Russia the mobile Bastion-P land-based coastal defense systems, including Yakhont anti-ship missiles capable of sinking large warships, including aircraft carriers.

Analysts have warned that an attempt to impose a no-fly zone such as the one against Moammar Gadhafi’s regime in Libya last year would be a very complicated task, which could result in serious losses. Syria’s air defense capabilities are much more extensive and sophisticated than Libya’s were.

And, since such an action would likely have to be carried out without consent by the U.N. Security Council, where Syria’s allies China and Russia wield vetoes, it would risk a major international confrontation if Moscow or Beijing decided to send radar ships to the area to provide the regime with advance notice of air raids.

This could mean a major international crisis that would probably end Moscow’s willingness to help resolve the dispute over Iran’s nuclear program, said Nick Witney, former head of the EU’s European Defense Agency.

“The Russians and Chinese won’t back down, and unilateral, unauthorized action by the West would risk an old-style East-West rupture over Syria. That would be a really big gamble in a tinderbox situation in the Middle East,” he said.

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Lekic reported from Brussels, Belgium.

Copyright 2012 The Associated Press.