Does any one know registration of the crashed aircraft and crash location ?
As far as I know, JS Air has following two Beech 1900 aircraft in its fleet.
Beech 1900C - registration AP-BJC
Beech 1900C - registration AP-BJD
Tail section of the crashed aircraft shown on local news channels.

According to news channels:
The aircraft took-off around 7:13AM Pakistan Time from Jinnah International Air Port (JIAP), Karachi.
The aircraft was going to Bhit Shah for a routine flight. Oil fields are located in/near Bhit Shah.
The aircraft experienced some technical fault with one of its engines.
The aircraft was allowed to return to Jinnah International Air Port (JIAP), Karachi.
The accident occurred around 7:15AM Pakistan Time.
A total of 21 souls were aboard the aircraft:
2 x pilots.
17 x Passengers.
1 x Airport Sercurity Force (ASF) guard.
1 x JS Air aircraft technician.
Names of two pilots are Capt. Naeem and Capt. Nauman. Capt Naeem was also Chief Pilot of JS Air. Both pilots were highly experienced according to JS Air spokesman.