NAIROBI - The US embassy in Kenya warned of a threat to American citizens in the country after Nairobi launched a cross-border operation against Islamist militants in Somalia.

The embassy in a note to US citizens living in or visiting Kenya said on Saturday that reprisal attacks could be directed at "prominent Kenyan facilities and areas where foreigners are known to congregate, such as malls and night clubs".

The statement said the embassy had taken measures to limit official US government travel to Kenya.

Kenya launched its boldest incursion yet into its anarchic neighbour six days ago after a wave of kidnappings against foreigners that Nairobi has blamed on the al Qaeda-linked al Shabaab militants.

The rebels have denied responsibility for the kidnappings and said Nairobi was using them as a pretext for an attack.  

The rebels have warned Nairobi to withdraw from its southern strongholds or risk bringing the "flames of war" into Kenya.