The statement casts doubt on interim Iraqi Prime Minister Iyad Allawi's assertion the trial could begin as early as October.
Allawi, who has urged Iraq's Special Tribunal to speed up proceedings against Hussein and his associates, has said he wants the trial to begin in October, before elections set for January.
But the US official on Friday: "If you are talking of a Saddam Hussein trial in 2004, I think that is remote. It's a very difficult schedule to meet.
"You are talking of a 25-year period with numerous and large crime bases and the investigation has to take place in all those crime bases. It is almost October," said the official, who is part of a team of Americans advising the tribunal.
Court officials have argued the tribunal, set up shortly after Hussein's government was toppled, needs at least a year to delve into tonnes of documents and prepare trials that will examine the deaths of hundreds of thousands of Iraqis.
Chalabi: Allawi is manipulating
the court to boost his popularity
The former tribunal head, Salim Chalabi, on Thursday accused Allawi of manipulating the court to boost his popularity and planning hasty show trials and executions.
The US official said he was unaware of attempts by the PM to pressure the tribunal, including moves to halt the investigation of former Baathist party members regarded as allies of Allawi, himself a former Baathist.
"Nobody from any quarter has pressured the tribunal to do anything. I have seen no evidence whatsoever that anybody is trying to halt any investigation," the official said.
He declined to speculate on a timetable for the trial, but said it would depend on the scope and the number of charges brought against the defendants.
The US blames the security
situation for the trial's delay
"For example, you have Hassan al-Majid. He is involved in many different crimes and in many different events. What are you going to charge him with? How many charges? Those are things they have to consider."
Chalabi, a US-educated lawyer once appointed by Washington to head the tribunal, has accused the Iraqi government of ousting him as part of a political vendetta. He has denied reports he resigned after murder charges were brought against him.
The official said Chalabi had been replaced as its chief administrator.
Hussein - deposed in last year's US-led invasion of Iraq - appeared before a US-appointed judge on 1 July and was informed he was being investigated for crimes against humanity.
Several top aides also appeared separately before the same judge. Pending trial, they are being held in a secret location by US forces under Iraqi jurisdiction.
However, it may be some time before they see a court again. Violence was delaying the trial of Hussein and his associates, the US official claimed, making it difficult for investigators to travel to exhumation sites and hampering the transport of bulldozers and other heavy digging equipment.
"Security has slowed the process tremendously," the official said. "We are talking of a lot of different massacre sites and we have to make decisions in terms of where we can go and where we can't."