"Make your choice today and withdraw all your forces," Ahmed Abdi Mohamed Godane, who goes by Mukhtar Abu Zubayr, said in a statement posted online late Wednesday, according to the AP.
Al-Shabaab, the al Qaeda affiliate based in Somalia, has taken responsibility for the four-day assault on an upscale Nairobi mall where at least 70 people were killed.
The latest attacks occurred on Thursday in Mandera, where two police officers were killed, and on Wednesday evening in Wajir, which is about 240 miles southwest of Mandera, according to the AP.
One person was killed there when gunmen opened fire and threw grenades.
The Nairobi mall attack has sparked concerns over al-Shabaab harboring broader ambitions than previously realized.
The group actively recruits Somali-Americans to join its ranks, and there have been unverified reports that Americans were among the assailants in Nairobi.