The US has imposed a travel ban on a senior Kenyan government official for obstructing efforts to rid the country of corruption.
Johnnie Carson, the US state department Africa chief, said he was considering bans on three other officials - but declined to release any names.
Kenya agreed to carry through reforms after 1,300 people died in post-election violence last year.
But the US believes some officials have deliberately been blocking the reforms.
The BBC's Will Ross, in Nairobi, says the US has to perform a balancing act when it comes to dealing with Kenya.
On the one hand Washington wants to exert pressure and help sideline some of Kenya's more unsavoury politicians. But the country is a vital ally in the region which the US relies on to help to dowse the flames of Islamist militancy in neighbouring Somalia, our correspondent says.
Mr Carson told reporters in Nairobi: "The US government has taken the decision to revoke the visa of a senior Kenyan government official."
Without revealing names, he described the politician as a "senior government official of influence".
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.
In July, former UN chief Kofi Annan passed the names of those accused of orchestrating the violence to the International Criminal Court in a sealed envelope.
The list, drawn up by a Kenyan judicial commission, has not been made public.
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