It is safe. NOKIA's AC-60E charger is also shown as fully compatible and I can confirm from my personal experience, that any charger up to 1.5 A is safe to use. If you use anything below 1A while using the phone have in mind it can still slowly discharge itself instead of charging it.
I have tried 2A one too just to see if it works, it charges too, but I can't recommend it, because I can't confirm phone is rated for max. 2A input and I am not using this one for charging.