Johnnie Carson, the state department chief who heads up the African desk, has revoked the visa of a high-ranking Kenyan official and is considering doing the same to three others.
Without naming names, Carson has said the move stems from reform obstruction in the wake of Kenya's post-election violence last year. From the BBC:
He said the individual had "obstructed the reform process, failed to end the cycle of impunity and has been an obstacle in the fight against corruption".
Last month Mr Carson sent a letter to 15 officials warning them they faced travel bans if they failed to support the "reform agenda".
He urged Kenya to strengthen its institutions and eradicate corruption to avoid more violence after the next election in 2012.
A power-sharing government was eventually set up after weeks of violence following the December 2007 election, but it has struggled to restore stability.
Rights groups blamed the police for many of the deaths in the riots.
International mediators have pressed the government to set up a tribunal to investigate the killings, but officials continue to miss every deadline they are set.