Somalia's Al-Qaeda linked Shebab rebels on Sunday warned Kenyan soldiers, as they crossed the frontier to attack insurgent strongholds, that they would feel the "pain of bullets."

"Kenya violated the territorial rights of Somalia by entering our holy land, but I assure you that they will return disappointed, God willing," said Sheikh Hassan Turki, a senior Shebab leader.

"Mujahideen fighters will force them to test the pain of the bullets."

Kenyan government spokesman Alfred Mutua said Sunday that its troops had entered southern Somalia to fight the Shebab, who it says are responsible for attacks on its territory, including the recent kidnapping four foreigners.

In response, the Shebab called on Somalis to rally and attack the Kenyan troops.

"I call on all Somalis to stand united against this blood-thirsty enemy that has crossed into our territories and the apostate Somali militants helping them," Turki added.

Kenya's assault comes a day after its Internal Security Minister George Saitoti branded the extremist Shebab rebels "the enemy" and vowed to attack them "wherever they will be".