The leader of the Somali Islamic militant group Shabab warned that Kenya will be hit with more deadly attacks like the Nairobi mall massacre if it does not withdraw its troops from southern Somalia.

In an audio message posted on Somalimemo.net, a sympathetic Web site, Mukhtar Abu Zubeyr, known as Godane, said Saturday's attack on the upscale Westgate mall was an act of revenge by Shabab for the Kenyan army's incursion into southern Somalia to eliminate Islamic extremists.

At least 72 people - including 61 civilians - were killed in the attack, which began Saturday and ended Tuesday after a four-day standoff.

"Remove your troops or prepare for a long, bloody war. Decide now and withdraw your troops from the Islamic regions of Somalia," Shabab's leader said.

"If not, be ready for more bloodshed in your territory, for economic collapse, and to see yourselves displaced," Godane said.

Islamic radicals have threatened to carry out reprisal attacks on Kenya since October 2011, when the Kenyan army invaded Somalia to assist African Union peacekeepers and the Somali government in their fight against Shabab.

That military intervention followed a spate of kidnappings allegedly carried out by Shabab on Kenyan soil.

On Thursday, at least two police were killed and two others were wounded in an armed attack in the northeastern Kenyan town of Mandera, which borders Somalia and Ethiopia.

That attack came a day after a similar incident Wednesday in the eastern Kenyan town of Wajir - 100 kilometers (60 miles) from the Somali border - left one dead.

Both Wajir and Mandera have been the target of various attacks that the Kenyan authorities blame on Shabab.

That group, which formally declared allegiance to Al Qaeda in 2012, has been battling Somalia's government since 2006 in a bid to establish a fundamentalist Islamic state in that East African nation.

Islamist militants control much of central and southern Somalia, which has been beset by civil war since military dictator Siad Barre was overthrown in 1991. EFE