Canadian officials have halted the transfer of prisoners to Afghanistan's intelligence service "more than one time," because of the possibility of torture, Canada's chief of defence staff said Sunday.

Gen. Walter Natynczyk, speaking at the end of the three-day Halifax International Security Forum, declined to offer details, saying additional information is expected as more witnesses speak before a special House of Commons committee.

"We indeed did stop the transfer more than one time," he said. "At the same time, I don't want to throw out more information. There's a process that's undergoing and I know that the witnesses will be called forward for that process and give their testimony."

Last week, Richard Colvin, a former Canadian diplomat in Afghanistan who says he delivered repeated warnings that prisoners handed over to Afghan authorities were being tortured, testified that all of the prisoners Canada handed over in 2006-07 were likely tortured.

Defence Minister Peter MacKay, speaking alongside Natynczyk at the security forum, said these were "very serious allegations," but there's no evidence to support them.

"What we have heard thus far cannot be substantiated. Not a single Taliban prisoner turned over by Canadian Forces can be proven to have been abused. That is the crux of the issue."

In responding to Colvin's testimony, Rick Hillier, former chief of the defence staff, said last week that the military, as well as the federal government, were concerned about torture in Afghanistan all along.

Hillier said the one credible report of torture the military received was in October 2007. That's when Canadian officials immediately halted transfers until new arrangements could be put in place to monitor Afghan prisons.

MacKay calls for international conference on piracy

Earlier at the Halifax forum, MacKay renewed calls for an international conference on piracy to discuss what to do with captured pirates and ways to come up with a co-ordinated response.

The minister said the situation in the Gulf of Aden, where Somalian pirates have been intercepting vessels, won't improve because of the patchwork response to piracy.

The European Union has negotiated a deal with Kenya to transfer pirates it captures to that African nation. But U.S. Admiral Mark Fitzgerald said Kenya's jails are overcrowded and its law enforcement officials are open to bribes.

"I don't believe we've yet had the first successful criminal trial of pirates and there have been several let-goes, so there are some issues there," he said.

The EU is talking to other African nations about similar deals, but MacKay said that presents other problems. He said some of those countries are using these talks to negotiate for more international aid.

Piracy experts say at last count, 13 ships and more than 200 passengers are being held for ransom by pirates.