Residents in Somalia say Kenyan troops have pushed farther into the southern part of the country, after crossing the border to fight against the militant group al-Shabab.

The residents said Monday the troops backed by armored vehicles were near the town of Qoqani.

An official with Ras Kamboni – a militia allied with the Somali government – told VOA that about 400 Kenyan soldiers crossed the border Friday and were in the Lower Jubba region Sunday, assisting Somali government forces.

The official, who spoke on condition of anonymity, said the Kenyans are advancing on Afmadow, a town controlled by al-Shabab.

Somali and Kenyan officials and witnesses confirm Kenyan forces have entered Somalia, following a buildup of troops on the border. They say the troops are backed by military aircraft. Residents of Liboi, a Kenyan town near the border with Somalia, said a Kenyan military helicopter crashed there Sunday evening.

Al-Shabab vowed to fight back against the Kenyans, calling on all Somalis to defend against “the enemy.”

Kenyan authorities blame al-Shabab for a series of recent kidnappings.

The kidnapping victims include a British tourist, a French woman and two Spanish aid workers. The Spaniards, with the medical aid group Doctors Without Borders, were kidnapped Thursday from the huge Dadaab refugee camp.

Saturday, Internal Security Minister George Saitoti announced Kenya would fight back against al-Shabab, “wherever they will be.”

Al-Shabab controls large sections of southern Somalia, including some areas near the border.

Kenyan forces have crossed into Somalia before, although this incursion appears to be on a larger scale.