Conducting creep test in LAMMPS

Dear all,
I want to simulate a creep test for nanotubes in LAMMPS. Therefore, I need to stretch the nanotube with a constant stress in a constant temperature, then, I have to evaluate the strain-time diagrams. My question is that how can I introduce stress (or force) to the both end of nanotube? I know how to perform a tensile test in a nanotube but I used fix move command, which is not useful here since I want to implement a constant pressure to the nanotube. For example, I used these lines for a tensile test:

fix 1 up move linear 0.0 0.0 0.001 units box
fix 2 down move linear 0.0 0.0 -0.001 units box

which, up and down are some atoms in top and bottom of the nanotube.

Thank you in advance.