NAIROBI, Kenya — The Islamic extremist group that killed scores of people at a Nairobi mall has now attacked two Kenyan towns near the Somali border, killing three people. The leader of the Somali group affiliated with al-Qaida said the attacks will continue until Kenyan troops are withdrawn from Somalia.

The leader of al-Shabab said in a message that there is no way Kenya can “withstand a war of attrition inside your own country.”

“Make your choice today and withdraw all your forces,” said Ahmed Abdi Mohamed Godane, who goes by his nom de guerre Mukhtar Abu Zubayr, in a new statement posted on the Internet late Wednesday. “Otherwise be prepared for an abundance of blood that will be spilt in your country, economic downfall and displacement.”

Al-Shabab said the Nairobi mall attack was not only directed at Kenya, but was also “a retribution against the Western states that supported the Kenyan invasion and are spilling the blood of innocent Muslims in order to pave the way for their mineral companies,” according to the statement from Godane.

Al-Shabab attacked Nairobi’s upscale Westgate mall Saturday and held it for four days in a siege in which at least 67 people were killed. Forensic experts from around the world, including the U.S., Britain, Germany and Canada, continued their work Thursday reconstructing events in the crime scene including by carrying out fingerprint, DNA and ballistic analysis.

Early Thursday, Al-Shabab fighters attacked the border town of Mandera, killing two police officers, injuring three others and destroying 11 vehicles, said regional police chief Charlton Mureithi.

Wednesday night, al-Shabab attacked the border town of Wajir, 390 kilometers (240 miles) southwest of Mandera. One person was killed and four wounded after a gunman opened fire and threw grenades.

Kenya has suffered many such attacks by al-Shabab along its 682 kilometer (423 mile) border with Somalia but they take on new significance following the Westgate Mall attack.