A video surfaced Thursday that appears to show a Syrian military helicopter shooting canisters of white phosphorus, a controversial substance that is a considered a chemical weapon by some military analysts.

In the video, a helicopter flying at a very high altitude above the town of Maraiya in Deir al-Zour province is shown firing a shell that explodes into smaller pellets and drops toward the ground leaving behind white smoke trails.

The video was posted on YouTube via an account used by the opposition. The authenticity of the footage is impossible to verify with the restrictions placed on media personnel by the Syrian government.

The footage comes at a time when international leaders have expressed great concern about the Syrian government using chemical weapons against its own people. On Monday, President Obama said there would be "consequences" if Syrian President Bashar al-Assad used chemical weapons. Secretary of State Hillary Rodham Clinton was even more explicit and called the use of chemical weapons a "red line."

The use of white phosphorus against military targets falls into a gray area within the Chemical Weapons Convention. Some observers say the use of white phosphorus for illumination in a military conflict is legal but that using it in a offensive capacity against military targets is illegal.

White phosphorus can cause severe chemical burns, and the smoke vapors can cause illness or even death. There also is a risk that white phosphorus residue can poison food stocks or water sources and lead to later poisoning.

In the video posted Thursday, the helicopter is clearly operating in the middle of the day, which makes it unlikely that the shells were being used for illumination. The U.S. military was heavily criticized for the use of white phosphorus shells in an offensive capacity during an operation to recapture the Iraqi city of Fallujah from insurgents in 2004.