Somali militant group al-Shabab is threatening attacks against Kenya unless the Kenyan government pulls its troops from Somali territory.
Kenyan soldiers and armored vehicles entered southern Somalia over the weekend in what the government called a "pursuit" operation against al-Shabab, which Kenya blames for a series of kidnappings near the Kenya-Somali border.
Speaking to reporters in Mogadishu Monday, al-Shabab spokesman Sheikh Ali Mohamud Rage said Kenyan troops have moved 100 kilometers into Somalia and carried out airstrikes in the country.
He said al-Shabab fighters will carry out reprisals inside Kenya unless Kenya withdraws its forces.
The exact size and location of Kenyan forces in Somalia is not clear. Residents have reported seeing columns of 30 to 40 armored vehicles. The troops were reported Monday to be near the village of Qoqani, which al-Shabab abandoned on Saturday.
Al-Shabab is calling on all Somalis to defend against what it calls "the enemy." Witnesses said Monday that convoys of al-Shabab fighters had left from the city of Kismayo to confront the Kenyan forces.
Kenyan authorities blame al-Shabab for a series of recent kidnappings that could jeopardize Kenya's status as a safe tourist destination.
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 Kenya's massive Dadaab camp for Somali refugees.
Kenyan forces have crossed into Somalia before, although this incursion appears to be on a larger scale.
A Kenyan military helicopter crashed on the Kenyan side of the border late Sunday, killing five Kenyan military personnel. Kenyan defense officials confirmed it was part of the contingent sent to fight al-Shabab but said the crash was caused either by pilot error or a mechanical malfunction.
Some information for this report was provided by AP, AFP and Reuters.