U.S. : Syrian Government maintains stockpile of Mustard Gas, Sarin and Vx

Source: CNN TV

All the ones above are nerve Gases.

Sarin and Vx are very similar. Here are nerve gas symptoms of both Sarin and Vx.
both are nerve gases, highly fatal and can kill within minutes (with a high enough dose)
first your pupils go down to a pinpoint with difficulty seeing
then headaches, excessive sweating nausea vomiting, convulsions, respiratory failure, paralysis and then death.

Mustard gas is different:

It doesn't necessarily cause death but instead Blisters on skin
Nausea and vomiting
blindness
then Cancer

So Mustard gas is not necessarily to kill soldiers but to make them unable to fight or function normally.

The new gas in todays report in Northern Syria is not one of these nerve Gasses or Mustard Gas and more likely resembles Phosgene or phosphorus gas or something like that.