By John Irish

PARIS (Reuters) - The chief prosecutor of the International Criminal Court (ICC) said on Wednesday that Kenya president-elect Uhuru Kenyatta would face a trial on charges of crimes against humanity but when that would happen was unknown.

Kenyatta, whose election earlier this month is being challenged by his rival, faces charges at the ICC over bloodshed in the aftermath of Kenya's 2007 election.

However, his lawyers said on Monday that the charges against him should be dropped after the case against a co-accused collapsed when an important witness's testimony was retracted.

"We will not drop the charges," prosecutor Fatou Bensouda told reporters in Paris. "It's not a question of if it goes to trial but when it goes to trial."

Bensouda said she was worried that bribery, intimidation and the Kenyan government's lack of cooperation seen in other cases could also impact the Kenyatta case.

"We still have difficulties with witness intimidation. This is ongoing," she told Reuters. "It's not stopping and I think it will get more serious."

Kenyatta's lawyers said the charges against him were clearly now based on hearsay after the witness retracted testimony in the case against former civil servant Francis Muthaura.

Kenyatta and Muthaura were among six suspects initially charged by ICC prosecutors with orchestrating violence after the 2007 election, when some 1,200 people were killed.

"Kenya is the most challenging situation that our office has had to deal with," Bensouda said.

When asked if the Kenyatta case could ultimately be in jeopardy due to bribes and intimidation she said: "We have a real risk of our witnesses being afraid to speak up and have anything to do with the ICC."

However, Bensouda said that unlike the Muthaura case, the ICC had more witnesses willing to testify against Kenyatta. "We continue to monitor our witnesses all the time," she said.

She added that the Kenyan authorities also were not cooperating sufficiently by providing only irrelevant documentation for cases.

"What the Kenyan government is claiming is that it has given us everything we have requested, but that's not correct. If you ask for 10 things you get four and they are not so relevant," she said.

(Editing by Michael Roddy)