diff --git a/cscripts/cspull.py b/cscripts/cspull.py
index 6968693..67a81ac 100755
--- a/cscripts/cspull.py
+++ b/cscripts/cspull.py
@@ -256,8 +256,8 @@ class cspull(ctools.csobservation):
                            onsrc=onsrc, onrad=onrad,
                            addbounds=True, seed=seed,
                            binsz=binsz, npix=npix, proj=proj, coord=coordsys,
-                           edisp=edisp, log=False, debug=self._logDebug(),
-                           chatter=self['chatter'].integer())
+                           edisp=edisp, nthreads=1, log=False,
+                           debug=self._logDebug(), chatter=self['chatter'].integer())
 
         # Determine number of events in simulation
         nevents = 0.0
diff --git a/cscripts/cssens.py b/cscripts/cssens.py
index e1d642a..2f24df0 100755
--- a/cscripts/cssens.py
+++ b/cscripts/cssens.py
@@ -418,7 +418,8 @@ class cssens(ctools.csobservation):
                                binsz=binsz, npix=npix,
                                log=self._log_clients,
                                debug=self['debug'].boolean(),
-                               edisp=self['edisp'].boolean())
+                               edisp=self['edisp'].boolean(),
+                               nthreads=1)
 
             # Determine number of events in simulation by summing the events
             # over all observations in the observation container
diff --git a/cscripts/cstsdist.py b/cscripts/cstsdist.py
index e43bedf..b357eaf 100755
--- a/cscripts/cstsdist.py
+++ b/cscripts/cstsdist.py
@@ -196,9 +196,10 @@ class cstsdist(ctools.csobservation):
         # only for event lists
         else:
             sim = obsutils.sim(self.obs(),
-                               seed  = seed,
-                               log   = self._log_clients,
-                               debug = self['debug'].boolean())
+                               seed     = seed,
+                               log      = self._log_clients,
+                               debug    = self['debug'].boolean(),
+                               nthreads =1)
 
         # Return simulated observation
         return sim
diff --git a/cscripts/obsutils.py b/cscripts/obsutils.py
index eb92f2c..adff62e 100755
--- a/cscripts/obsutils.py
+++ b/cscripts/obsutils.py
@@ -28,7 +28,7 @@ import cscripts
 # ===================== #
 def sim(obs, log=False, debug=False, chatter=2, edisp=False, seed=0,
         emin=None, emax=None, nbins=0, onsrc=None, onrad=0.2, addbounds=False,
-        binsz=0.05, npix=200, proj='TAN', coord='GAL'):
+        binsz=0.05, npix=200, proj='TAN', coord='GAL',nthreads=0):
     """
     Simulate events for all observations in the container
 
@@ -74,6 +74,8 @@ def sim(obs, log=False, debug=False, chatter=2, edisp=False, seed=0,
         Projection for binned simulation
     coord : str, optional
         Coordinate system for binned simulation
+    nthreads : str, optional
+        Number of parallel processes for On/Off spectra computation (0=all available CPUs)
 
     Returns
     -------
@@ -133,6 +135,7 @@ def sim(obs, log=False, debug=False, chatter=2, edisp=False, seed=0,
             phagen['stack']       = False
             phagen['inexclusion'] = 'NONE'
             phagen['bkgmethod']   = 'REFLECTED'
+            phagen['nthreads']    = nthreads
 
             # Optionally open the log file
             if log:
@@ -780,6 +783,8 @@ def get_onoff_obs(cls, obs, nthreads=0):
         cscript class
     obs : `~gammalib.GObservations`
         Observation container
+    nthreads : str, optional
+        Number of parallel processes for On/Off spectra computation (0=all available CPUs)
 
     Returns
     -------