Syria conflict: 'Chemical attacks kill hundreds'

Underway? They are situated in Damascus, but thats about it.

They need to get access to be able to verify if Nerve gas (and that is what is alleged to have been used) was used. Which it is almost certainly the case.

So far, no access for them.

And as such is the situation they need the approval of the government and the various rebels, also whichever specific armed group holds that particular area and whichever specific armed groups hold the neighbouring areas.
It takes a lot of negotiating with a lot of groups to allow a bunch of foreigners to go poking around the neighbourhood in any degree of safety.