US Ambassador Robert Godec said that if the electoral commission felt it was not ready for Thursday's poll, it should ask the courts for a delay.
Kenyans on Thursday are due to hold their second presidential election this year, after the Supreme Court overturned the results of an August 8 vote citing "irregularities and illegalities".
Last week one of the IEBC's seven commissioners, Roselyn Akombe, fled the country and quit, saying the election could not be credible.
Western nations on Monday warned starkly of Kenya's "deteriorating political environment" ahead of a presidential election re-run and urged the major parties to halt incitement and verbal attacks on institutions.
US Ambassador Robert Godec said that if the electoral commission felt it was not ready for Thursday's poll, it should ask the courts for a delay.
"We would be fine with that," he said.
Godec read a statement from 20 Western envoys, including the 28-nation European Union, calling for political rivals to unite to allow a credible election to take place, after weeks of political drama that has gripped and divided the nation.
Kenyans on Thursday are due to hold their second presidential election this year, after the Supreme Court overturned the results of an August 8 vote citing "irregularities and illegalities".
The ruling was a rare victory for veteran opposition leader Raila Odinga, who has since refused to take part in the re-run, accusing the Independent Electoral and Boundaries Commission (IEBC) of failing to make required reforms.
He has called for mass protests on election day, and wrote on Twitter that his supporters would picket outside IEBC offices on Tuesday and Wednesday.
The diplomats slammed incidents in which poll officials have been attacked and their training interrupted by opposition protesters.
Violence
"Attacks on IEBC staff must cease now. No one is obliged to stand for office, or to vote if they do not wish to," said Godec.
"But no one should use violence or intimidation to disrupt the right of others to vote or to participate. Doing so is profoundly undemocratic, and leaders must tell their supporters to refrain from such actions."
He said the diplomatic community has been working to encourage Odinga to rejoin the election and continues "to hope they will do so in light of the progress made at the IEBC."
While the IEBC has not conceded to several major demands from Odinga, such as the firing of certain staff, and a change in providers of ballot papers and election technology, Godec said they had made several important changes to appease the opposition.
He also said attempts by the ruling party to change certain electoral laws ahead of the re-run were "unhelpful" and urged Kenyatta not to sign the amendments.
"Unfortunately, the deteriorating political environment is undermining preparations for the new presidential election," Godec said, in a statement in the name of the 20 envoys.
"Inflammatory rhetoric, attacks on institutions, and growing insecurity all make holding a credible and fair poll more difficult... it is dangerous, and it must stop."
Akombe
The statement was endorsed by Australia, Austria, Belgium, Britain, the Czech Republic, Denmark, the European Union, Finland, France, Germany, Ireland, Italy, Netherlands, Norway, Portugal, Slovakia, Spain, Sweden, Switzerland and the United States.
Last week one of the IEBC's seven commissioners, Roselyn Akombe, fled the country and quit, saying the election could not be credible.
The body's chairman, Wafula Chebukati, warned he could not guarantee a free and fair poll, and its controversial chief executive, Ezra Chiloba, suddenly went on three weeks' leave.