UNDATED (CNN) -- Just how big are the mortars being fired into Homs by Syrian forces? Some think they are using one of the world's heaviest.

We are now learning one reason why the death toll is climbing steadily in Syria. A researcher with Human Rights Watch says a video, first flagged by Christian Science Monitor, proves that the Syrian military is using one of the heaviest mortars in the world to shell the city of Homs.

While CNN cannot verify this video independently, it appears to show two men holding the fanned tails of the exploded ordnance. And there's file video of these massive mortars in action on the range.

Lobbed from the Russian-made "Tulpan" mobile firing platform, the 240 millimeter rounds carry more than 30 kilograms of explosives, with a range of nearly 20 kilometers.

So here's the scale of this massive weapon. The Tulpan artillery shell is roughly one and a half meters long. That's about as tall as an ordinary compact car.

A single artillery vehicle can fire one of these devastating rounds every minute. So let's look at the epicenter of the civilian uprising in Syria, the country's third largest city, Homs, home to about a million people.

Parts of the city are a warzone. These highlighted areas are majority Sunni neighborhoods where most of the protests have been taking place, and also where we're seeing the worst of the fighting and the shelling by government forces.

The rebel controlled neighborhood of Baba Amr has been under heavy bombardment for nearly three weeks straight. Other parts of the city remain relatively calm. There are several majority Alawite neighborhoods in the southeast of Homs.

These are wealthier neighborhoods which have seen little or no protests. By the way, Syria's President Bashar Al-Assad is also an Alawite Muslim.

It's worth noting, Homs is important to the Syrian government because its home to one of the country's two oil refineries.

And preserving that oil supply is crucial to the Assad regime.