Col. Cyrus Oguna, an army spokesman, said another 20 of the militant Islamic fighters have defected in recent weeks, the BBC reported.
"We will keep hitting them until their spine is completely broken ... and we will relish that moment," Oguna said.
"The resistance from al-Shabaab is crumbling, there is division within the al-Shabaab leadership."
Al-Shabaab leaders responded by promising to defeat the Kenyan army.
The Kenyan military intervention was "rapidly rolling down a path towards an ignominious end endured by all previous invaders," al-Shabaab officials said on Twitter.
Meanwhile, al-Shabaab has been enlisting young boys in Afyoye, a small town about 18 miles from Mogadishu, Shabelle News reported.
Local residents said al-Shabaab vehicles with microphones were seen Friday patrolling the streets to recruit young boys.