Syrian rebels have acquired as many as 40 shoulder-fired missile systems in recent weeks to counter assaults by Syrian military aircraft, introducing a possibly decisive new weapon into the conflict, Western and Middle Eastern intelligence officials say.

The potential impact of the missiles on the 20-month-old civil war was demonstrated Tuesday with the dramatic downing of a Syrian helicopter, blasted from the sky near Aleppo by what military experts say was almost certainly a portable anti-aircraft missile.

The Obama administration has steadfastly opposed arming Syrian opposition forces with such missiles, warning that the weapons could fall into the hands of terrorists and be used to shoot down commercial aircraft.

But intelligence officials who closely track the flow of arms into Syria say rebels have acquired dozens of the devices in recent weeks and are using them with increasing effectiveness against Syrian helicopters and military jets.

At least some of the missiles were supplied by Qatar, which has supplied most of the weapons smuggled to Syria's rebels across the Turkish border, according to two Middle Eastern intelligence officials briefed on the matter. The officials, along with others interviewed, spoke on the condition of anonymity because of the sensitive nature of the information.

More than 30,000 people have died and an estimated 400,000 refugees have fled Syria since the civil war began. The opposition forces, which have struggled against the superior arms and the air power of the Syrian military, have pleaded for heavier weapons from the international community.

Together with other anti-aircraft weapons seized from Syrian army depots, the MANPADS - the common name for man-portable air defense systems - provide the rebels with a powerful defense against the airstrikes that are seen as critical to the regime's defense.

While the missiles are seen as a potential game-changer in the fight against Syrian President Bashar Assad, their arrival has evoked fear and dismay among Syria's neighbors as well as Western countries, including the United States. In the hands of terrorists, the easily concealed missiles could be used to blow up commercial jets, weapons experts and intelligence officials say.

"It should be worrying to everyone," said one of the Middle Eastern intelligence officials, whose government closely monitors events in Syria. "When Assad is finished, terrorists could end up with these, and commercial flights would be at risk."

The renewed focus on anti-aircraft missiles was prompted by video footage that appeared to show a Syrian helicopter and warplane being shot from the sky in separate incidents.

On Wednesday, opposition activists reported the downing of a military aircraft near the Turkish border, but the circumstances were unclear.

The helicopter that crashed Tuesday, which military experts described as a Russian-built Mi-8 transport helicopter, appeared to have been struck by a large projectile as it flew over a suburb of Aleppo, according to an amateur video posted by Syrian anti-government activists. Military experts who examined the video said the missile's size, smoke trail and targeting were all consistent with a MANPADS attack.

"The helicopter was hit in the engine-compartment area, which suggests a heat-seeking missile," said Jeffrey White, a former analyst for the Pentagon's Defense Intelligence Agency. While the type of missile could not be determined conclusively from a video, the evidence points to MANPADS, he said.

Based on reviews of imagery posted on opposition websites, White said the number of MANPADS controlled by the rebels may be larger than the figure of 30 to 40 suggested by regional intelligence services. Reports and occasional videos of anti-aircraft missiles in Syria have been surfacing since late summer. Recent rebel attacks on aircraft appeared to involve portable missiles.

"We've seen videos with people showing off boxes of them," White said.

He noted that some of the rebels acquisitions could have come from Syrian army posts that have been overrun. Intelligence officials said MANPADS were stored at army armories throughout the country before the start of the civil war in March 2011.

Both Qatar and Saudi Arabia have provided extensive help for Syria's rebels in recent months, providing cash and weapons for a guerilla army that has received only non-lethal aid from Washington and Western Europe.

E-mails and phone messages requesting comment from Qatar's diplomatic missions in New York and Washington were not returned. U.S. intelligence officials declined to comment on the reports of MANPADS in the hands of Syrian rebels.

Shoulder-fired anti-aircraft missiles have served has equalizers in previous insurgencies. Afghanistan's mujaheddin fighters effectively used U.S.-supplied FIM-92 "Stinger" missiles against vastly superior Soviet forces during that country's civil war in the 1980s.

Then, as now, U.S. officials and independent experts warned about the potential consequences of allowing the highly mobile weapons to be given to fighters who could not guarantee their future disposition.

"The proliferation of MANPADS in Syria is a concern because of the danger that they will be acquired by terrorists or other armed groups and used against civilian aircraft," said Matt Schroeder, a senior analyst for the Federation of American Scientists in Washington.