Syrian President Bashar Assad should “absolutely” be tried for crimes against humanity and war crimes, the top U.S. official in charge of such issues told Yahoo News.

And the United States has, in a way, already started to build a case against him by training Syrians to collect information and analyze incidents on the ground in the country’s bloody civil war — evidence to use against Assad if that day comes.

“Enormous crimes have been committed that can be traced directly to the highest levels of the Syrian regime,” Stephen Rapp, the U.S. ambassador-at-large for war crimes issues, said in an exclusive interview Wednesday.

“Without question, there are credible, very credible allegations of crimes against humanity, murder, rape, mutilation, other crimes,” Rapp said. “And, of course, since we now have a civil war these become war crimes as well.”

Not only should Assad “absolutely” face trial, Rapp said, but the quantity and quality of the evidence against him means his prosecutor “would have one of the better cases that we’ve seen at the international level against a chief of state.”

Rapp warned that the process would face major hurdles (like opposition from Russia) and could take a long time. But he underlined that, when the time comes, there will be no lack of evidence, since it’s already being collected and stored in a special facility in Europe.

“We’re working with Syrian groups, educating them, training them in how to find this information, identifying it, and measuring shell holes, and determining what direction the shells came from, other things like that ” Rapp said.

“And we’ve actually, working with them, obtained more than 200,000 pages of documents which is now being analyzed and collected in a center, which the United States government helped establish that’s in Europe,” he said. “They’re developing the information that’ll be available when justice is possible.”

Rapp said he has been doing extensive diplomatic work to solicit international support for Washington's preferred approach — a Sierra Leone-style war crimes court that would be a blend of national and international staff with the support of regional powers.

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“Our preference is to work with Syrians, with people in the region, with international players and work toward establishing some kind of hybrid, or mixed court,” he said. “That will take time, (but) it could be begun even before there was an end to the conflict.”

But “at the end of the day, it’s not something we can do alone.”

Rapp also told Yahoo News that notorious warlord Joseph Kony is “in a box,” thanks to international efforts; explained how the United States is using rewards of up to $5 million to get information on Kony and other fugitives; and explained how he went from being a prosecutor in Iowa to the government’s top war crimes official. The full interview will be posted on Yahoo News on Thursday.

The diplomat’s declaration that Assad should face trial came after weeks in which top officials, starting with President Barack Obama, have accused the Syrian leader of crimes but stopped short of saying he should be prosecuted.

In his Sept. 11 address to the nation on Syria, the president said the international community regards Assad’s alleged use of chemical weapons to slaughter civilians as “a crime against humanity, and a violation of the laws of war.”

But a day later, White House press secretary Jay Carney ducked and dodged when asked whether Obama had decided that Assad was a war criminal who should face prosecution by the International Criminal Court.

“That's obviously a distinct matter that was not a focus of the president’s remarks,” Carney told reporters. “It’s a not a focus of the discussion right now about using military force in response to the use of chemical weapons or pursuing the diplomatic opening that is currently being explored, so I would leave that for discussions in the future.”

U.S. officials had always hinted at possible prosecution of the Syrian strongman and his close advisers. A senior Obama aide told reporters on an Aug. 30 conference call that top aides to Assad “should consider the decisions that they make.”

“To associate with their regime, they would commit a violation of international law like the use of chemical weapons,” the official said. “So those around President Assad I think should consider the type of accountability that they will face in the long run from the international community for this use.”