Reports from southern Somalia say Kenyan troops have entered the area, though Kenya's defense minister denies the claim.

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

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

Other officials and witnesses also say Kenyan forces have entered Somalia, backed by military aircraft, following a buildup of troops on the border.

Kenya's Defense Minister Yusuf Haji told VOA earlier that Kenya is not sending troops into Somalia. But a government spokesman, Alfred Mutua, said Kenyan troops have crossed the border to pursue al-Shabab.

Kenyan authorities blamed the group for a series of recent kidnappings.

The 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 massive Dadaab refugee camp.

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.

In a related development, Al-Shabab withdrew its forces Saturday from the town of Qoqani in Lower Jubba, after clashes with government troops and Ras Kamboni.