Extremist gunmen threatened to use force and carry out bombed attacks against university students in Yemen's southern city of Aden, if they do not observe segregation of the sexes on campus, witnesses said on Monday. The warning came as a suicide bomber killed two pro-government fighters in a blast that targeted a checkpoint in the center of Yemen's second city, a security official said.Students said armed militants distributed leaflets containing threats that were signed by ISIS terrorists in at least three departments of the University of Aden.The leaflets banning mixed gender education also prohibited music and demanded that students perform collective prayers on campus, they added.They set a Thursday deadline for the demands to be met.Otherwise they threatened to carry out car bomb and petrol bomb attacks.The authenticity of the leaflets signed by the Aden and Abyan branch of ISIS could not be immediately verified.Southern fighters allied with President Abedrabbo Mansour Hadi and supported by Saudi-led coalition forces managed in July to push Shiite rebels out of the port city of Aden.But the extremists, including Al-Qaeda and ISIS, appeared to have gained ground in the southern city, where the terrorists are now visibly present.Also on Monday, a man wearing an explosives belt detonated his bomb at a checkpoint in the center of Aden, killing two southern fighters and wounding a third, a security official said.Al-Qaeda, active across several parts of Yemen, has exploited the collapse of central authority during a 2011 uprising that ousted veteran president Ali Abdullah Saleh.