Kenyan Foreign Minister Moses Masika Wetangula told the BBC that Kenyan forces crossed the Somali border to take on al-Shabaab. Ali Mohamud Rage, a spokesman for the group, told the broadcaster in response that al-Shabaab was ready for a fight.
"We have fought against governments older and stronger than Kenya and we have defeated them," he said.
Wetangula said Nairobi was acting at the request of the Somali transitional government but added the deployment could be seen as an act of self-defense.
Al-Shabaab was allegedly behind the abduction from Kenya last week of two Spanish women working for humanitarian group Doctors Without Borders.
African Union officials recently told the BBC pro-government forces had wrestled control of the Somali capital Mogadishu from the terrorist group.
Al-Shabaab claimed responsibility for an early October suicide attack in Mogadishu that left more than 70 people dead in an assault that targeted Somali officials and soldiers from a peacekeeping mission in the area.
With violence and kidnappings threatening the Somali border with Kenya, London in early October warned against all but essential travel to within 90 miles of the Somali coast.