Kenyan forces said they were moving toward Somalia at the request of the transitional government. The deployment follows a series of kidnappings and a warning from the British government to avoid traveling near the border region.
Al-Shabaab was quoted by CNN International as saying Tuesday that it considered the Kenyan deployment "an affront to Somalia's territorial sovereignty."
Kenya announced it was invoking its right to self-defense in response to an increase in violence in neighboring Somalia.
The African Union last week announced that al-Shabaab had lost control of key positions in Mogadishu. The militant group, aligned with al-Qaida, 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.
The group was allegedly behind the abduction from Kenya last week of two Spanish women working for humanitarian group Doctors Without Borders.
Al-Shabaab had controlled most of Mogadishu as the transitional government struggled to exert its authority. It maintains a heavy presence in much of Somalia's south.