Kenyan forces have crossed into Somalia to fight the al Shabaab group which is accused of kidnapping foreigners.
Al Shabaab has said it would cross into Kenya if the government in Nairobi did not withdraw its forces.
foreign minister Moses Masika Wetangula said Kenya was defending itself, after a spate of kidnappings by al-Shabab.
The BBC says witnesses described dozens of military vehicles crossing the border, backed by planes and helicopters.
Several Westerners have been seized in Kenya by Somali groups and reportedly taken into Somalia.
Two Spanish aid workers from Medicins Sans Frontieres were abducted from the Dadaab refugee camp on Thursday.
A British woman and a French woman have been kidnapped from remote beach resorts over the past month.
There have been conflicting reports about possible perpetrators of the kidnappings - with some claiming al-Shabab carried them out and others saying pirate gangs could be the attackers.
