The US embassy in Nairobi warned on Saturday of an "imminent threat" of attacks possibly targeting foreigners, one week after Kenyan forces crossed into Somalia to hunt down al Shabaab fighters.
The embassy cited "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 has taken measures to limit official US government travel to Kenya and US citizens should take this information into account when planning travel and consider deferring travel at this time," the embassy added, in a statement provided by a State Department spokesman in Washington.
The statement did not specify who might carry out such attacks.
The State Department issued a travel warning for Kenya in December that remains current and warns of "continuing threats from terrorism and the high rate of violent crime."
Last week, Kenya sent troops across its border with Somalia to hunt Shabaab insurgents it blames for the abductions of a British tourist, a disabled French woman who has since died in captivity and two Spanish aid workers.
Kenya has not said how many of its troops are deployed, but analysts estimate the number at between 2,000 and 3,000.
The troops are advancing in a three-pronged movement towards the Shabaab-held port city of Kismayu but their advance has been slowed by bad weather.
The radical Islamist Shabaab, who deny kidnapping foreigners, have repeatedly warned of bloody retaliation.
The latest threats came on Saturday from the Shabaab's leader Mohamed Abdi Godane:
"The Islamic regions in Somalia are all on high alert to prepare for the open war that is our response to the incursions by some neighbouring countries who are taking part in the global Christian invasion against Somalia," he said.
In response to the warnings, officials have beefed up security in Nairobi's central business district.
On Saturday security personnel moved bystanders further away from the entrance to the Hilton hotel and conducted identity checks on people who looked as if they could be Somali.
Kenya has a large Somali population, made up both of Somali nationals, many of whom have fled war and famine back home, and of ethnic Somalis who are Kenyan citizens.
In 1998, 224 people were killed in truck bombings at the US embassies in Kenya and Tanzania.