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"It is with shock and sadness that I learned the death of some 80 paramilitary cadets killed in a terrorist attack," Rasmussen wrote in a letter to President Asif Ali Zardari.
"This attack is a stark reminder that Pakistan and Allies must stand united to defeat the scourge of violent extremism," he added.
The death toll rose to 89 following Friday's suicide bombing on police training centre in Shabqadar. Around 140 people were also wounded, 40 of them critically.
Pakistan's Taliban has claimed responsibility for the attack, saying it was to avenge the death of Al-Qaeda leader Osama bin Laden at the hands of US forces two weeks ago.