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.

Kenyan government spokesman Alfred Mutua told CNN about the advance this week, which marks a dramatic shift in security tactics for the east African powerhouse.

“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 U.S. Embassy said in an emergency message.

It added that it has taken measures to limit official U.S. government visits and that citizens should consider deferring travel to Kenya.

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.

The group has said it considers the Kenyan forces crossing the border as “an affront to Somalia’s territorial sovereignty.” It previously threatened to “come into Kenya” if Kenyan forces did not leave Somalia.

Recent abductions of tourists and aid workers in Kenya have heightened tensions.

On September 11, armed bandits broke into a beachfront cottage where Britons Judith and David Tebbutt, both in their 50s, were staying. David Tebbutt was shot dead while trying to resist the attack. His wife was grabbed and spirited away on a speedboat, and is believed to have been taken into Somalia.

On October 1, pirates made another cross-border raid, this time snatching a French woman in her 60s, who used a wheelchair and was believed to be in bad health, from a holiday home on Manda Island where she lived part of the year. She later died, likely because of the kidnappers’ refusal to give her medicine, according to the French Foreign Ministry.

And earlier this month, gunmen abducted two Spanish workers from the medical charity Doctors Without Borders from the Dadaab refugee complex, about 80 kilometers (about 50 miles) from the Somali border.

Al-Shabaab has denied responsibility for the abductions.