Nairobi, Kenya (CNN) -- The U.S. Embassy in Kenya warned American citizens Saturday of an "imminent threat of terrorist attacks" after Kenya sent troops across the border into Somalia to pursue suspected Islamic militants from Al-Shabaab.
"This is to inform U.S. citizens residing in or visiting Kenya that the U.S. Embassy in Kenya has received credible information of an imminent threat of terrorist attacks directed at prominent Kenyan facilities and areas where foreigners are known to congregate, such as malls and night clubs," the embassy said in an emergency message.
Kenyan government spokesman Alfred Mutua told CNN on Sunday about the advance into Somalia, which marks a dramatic shift in security tactics for the east African powerhouse.
Al-Shabaab, which is linked to al Qaeda and has been designated a terrorist organization by the United States, is fighting to impose its own interpretation of Islamic law, or sharia, on Somalia
Recent abductions of tourists and aid workers in Kenya have heightened tensions.
This is no joke! Al-Shabab has been responsible for the latest bombings in Somalia, contributed to one of the worst famines in Africa.
|Al-Shabab threatens to attack Kenya|
|Fighters from Al-Shabab will attack Kenya unless it withdraws its troops from Somalia, a spokesman for the group has warned. Analysts say the group will try to follow through on its threat.|
Hundreds of Kenyan troops entered Somalia on Sunday, backed by helicopters and tanks; officials in Somalia also said that jets had bombed al-Shabab camps, though Nairobi would not confirm the jets were theirs.
The troops are reportedly advancing towards Afmadow, a town about 120km east of the border. Kenya's government says they have been ordered to attack al-Shabab bases in southern Somalia. The group has been linked to several recent kidnappings in Kenya.
Sheikh Ali Mohamud Rage, a spokesman for Al-Shabab, told Kenya to withdraw its troops.
"Kenyan troops have entered 100 kilometres into Somalia and their planes bombarded many places and killed residents," Rage was quoted by the Reuters news agency on Monday. "We shall come into Kenya if you do not go back."
His claims about the Kenyan offensive could not be independently verified, and Kenya's defence ministry did not respond to a request for comment. Somalia's transitional government has endorsed the raid: Nur Ahmed, the Somali ambassador in Nairobi, said Kenya "has a right to defend its people."
'The restraint would be gone'
Al-Shabab has attacked Kenya's neighbours before - it killed more than 70 people in twin bombings last year in Kampala, the Ugandan capital - but the group has been reluctant to attack Kenya, despite repeated threats, because it feared reprisals from the Kenyan army.
But with that army already inside Somalia, analysts say al-Shabab might be more likely to carry out an attack inside Kenya's borders.
"Shabab was fearful that it would trigger a robust Kenyan response," EJ Hogendoorn, an analyst at the Brussels-based International Crisis Group, said. "If in fact you've already triggered that response, then to some degree the restraint on Shabab would be gone."
Sunday's Kenyan raid is largely a response to a recent spate of kidnappings in the country's coastal regions and along the border with Somalia.
Two Spanish aid workers were kidnapped last week, and their driver was wounded, in an attack on Ifo refugee camp in Dadaab, which houses tens of thousands of Somali refugees. Earlier this month, a French woman was kidnapped from her home in Manda, on Kenya's northern coast.
And last month, a British tourist was kidnapped from the resort of Kiwayu; her husband was shot dead.
"This is affecting their tourist industry and the security of international staff working for various aid agencies," Hogendoorn said. "If this affects regions along the coast, it's an enormous hit on their economy."
An Ethiopian parallel?
This is hardly the first Kenyan incursion into Somalia: Troops have pursued al-Shabab fighters across the border before. But those raids are usually brief and involve limited numbers of troops.
Sunday's incursion, on the other hand, was much larger, and it was announced in advance: George Saitoti, Kenya's interior security minister, held a press conference on Saturday and promised Kenyan forces would attack al-Shabab "wherever they will be."
Roger Middleton, a regional analyst at Chatham House, drew a parallel between the Kenyan incursion and Ethiopia's 2006 invasion of Somalia.
"It's a substantial and longer-lasting intervention, and if that's the case, the first thing that springs to mind is, Ethiopia tried this and it did not turn out too well," Middleton said.
The Ethiopian invasion, and the years-long occupation that followed, is widely credited with strengthening al-Shabab, which was able to portray itself as a resistance movement fighting an occupying power.
And the group is already attempting to frame the Kenyan incursion in the same way: A radio station controlled by al-Shabab asked Somalis if they were ready to "live under Christians" and urged them to fight.
"Get out of your homes and defend your dignity and religion. Today is the day to defend against the enemy," an al-Shabab official said in the radio broadcast, according to the AFP news agency.