Kenyan troops are advancing on a strategic rebel stronghold in southern Somalia as part of a cross-border military operation against al-Shabab militants in Somali territory.

Kenyan ground forces, backed by heavy air strikes, approached the rebel-held town of Afmadow on Tuesday, as al-Shabab rushed in reinforcements to the city.

On Sunday, Kenya declared war on al-Shabab, sending troops into southern Somalia in what it called a “pursuit operation” against the al-Qaida-linked militant group, which it blames for a series of kidnappings near the Kenya-Somali border.

Al-Shabab vowed revenge against Kenya on Monday, promising to carry out attacks inside Kenya unless its government withdraws its troops from Somali territory.

Meanwhile, Kenyan police said Tuesday they have arrested two British citizens suspected of being linked to al-Shabab. British officials are working with Kenyan authorities to determine why the men are being held.

The exact size and location of Kenyan forces in Somalia is not clear. Residents have reported seeing columns of 30 to 40 armored vehicles.

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 fled the rebel-controlled port 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.