The US has announced it is suspending funding for Kenya's anti-corruption efforts following the resignation of the country's top anti-graft official.
Githongo has spent years fighting corruption
John Githongo, who stepped down while on a visit to the UK, is thought no longer to believe in President Mwai Kibaki's commitment to the cause.
US ambassador William Bellamy said the amount to be withheld for 2005 and 2006 was about 200m shillings ($2.5m).
Diplomats and the media have expressed sadness at Mr Githongo's departure.
Mr Bellamy said John Githongo's departure was "a severe setback", while the UK envoy Sir Edward Clay said he left a large hole.
President Kibaki was elected in 2002 on a pledge to clean up Kenya.
He appointed Mr Githongo, a former head of the Kenyan branch of watchdog Transparency International, just a month after his inauguration, to oversee his fight against corruption.
But last week, Sir Edward said that "massive looting" was continuing under Mr Kibaki.
Although Mr Githongo has not spoken publicly about his reasons for resigning, he is understood to have lost faith in the president's personal commitment to ending graft.
Victory for corruption?
Mr Bellamy announced the US government was to suspend all it funding to Kenyan anti-corruption efforts for the next two years on Tuesday.
"We are eager to work with Kenya to improve governance, and we are in a position to be generous in this regard," he said.
"But we cannot be helpful when that government isn't serious or, worse, that government is the source of the problem."
Mr Bellamy had earlier told Kenya's Daily Nation newspaper that the US authorities were concerned that Mr Githongo's resignation followed "new, more extensive information on possible corrupt practises within the government of Kenya".
He urged the authorities to act quickly to ensure corrupt officials "have not won the day".
"Another strong, independent and effective anti-corruption crusader must be appointed immediately in order to restore international donor confidence in the current government," he added.
The East African Standard newspaper raises fears of "unfettered looting" of Kenya, now that the biggest obstacle to this has been removed.
The chairman of the Law Society of Kenya, Ahmednassir Abdullahi, who resigned last year from the Kenya Anti-Corruption Commission, told the paper: "Mr Githongo has seen rather late what we saw a long time ago - that the government is not committed to the fight against corruption."
Calling Mr Githongo's resignation "a complete disaster for the president personally, the government as a whole and the country in general", the Standard urged Mr Kibaki to remove corrupt officials.
Mr Githongo's resignation letter offered no explanation for his decision.
Donors initially welcomed Mr Kibaki's efforts to stamp out corruption and restored aid, which had been cut off under former President Daniel arap Moi.
But some diplomats estimate that Kenya has lost some $1bn in corruption over the past three years.
A Transparency International survey of perceived corruption rated Kenya 122nd out of 133 countries in 2003.