US warning of “imminent threat” posed by Al-Shabaab in Kenya (VIDEO)

American citizens in Kenya were warned Saturday of an "imminent threat of terrorist attacks" after Nairobi sent troops across the border into Somalia to pursue the Al-Qaeda linked Al-Shabaab Islamic militants.

American officials have branded Islamist militants in Somalia a serious threat to the United States, however the offensive by Kenya, one of the closest American allies in Africa, apparently took the U.S. by surprise.

An official told the New York Times that Washington had "zero" information on the operation before it started a week ago.   

According to the latest Associated Press report, Kenyan troops had closed in on the rebel-held Somali town of Afmadow and within fighting distance of the Al-Shabaab militants. 

Afmadow is controlled by Al-Shabaab, designated a terrorist organization by the United States, which is fighting to unseat the country's weak U.N.-backed government and impose its own interpretation of Islamic law on Somalia.

(GlobalPost reports: Somalia's Al Shabaab militants claim to have killed dozens of African Union peacekeepers)

Meanwhile, the U.S. Embassy said in an emergency message reported by CNN:

"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."

U.S. citizens and officials should consider deferring travel to Kenya, it said.