At Iraq's request, the US military recently transferred hundreds of metric tons of yellowcake uranium from Iraq to Canada in a secret, weeks-long operation, a Pentagon spokesman said Monday.

The 550 metric tons of uranium, which was sold to a Canadian company, was moved by truck convoy to Baghdad's "Green Zone," then flown by military aircraft to a third country where it was put on a ship for Canada, said Bryan Whitman, the spokesman.

"The operation was completed over the weekend, on Saturday," Whitman said.

The yellow cake was discovered by US troops after the 2003 US invasion of Iraq at the Tuwaitha Nuclear Research Facility south of Baghdad, and was placed under the control of the International Atomic Energy Agency.

Yellowcake is a form of processed uranium ore that can be used to make fuel for nuclear reactors, or if further enriched as fuel for nuclear weapons.

Whitman said the Iraqi government asked the United States for help in selling and transferring the uranium to another country.

Cameco, a Canadian company, agreed to buy the yellowcake for a reported sum in the tens of millions of dollars.

The cost to the United States of transferring the uranium came to about 70 million dollars, Whitman said. He said the Iraqi government has agreed in principle to repay part of the transfer costs.

The US State and Energy Departments also played roles in the operation, Whitman said. But the Defense Department took the lead in carrying out the transfer, he said.

It took 110 shipping containers to carry the drums loaded with yellowcake, he said.

They were transferred by convoy from Tuwaitha to a secure facility within the Green Zone, the international zone, Whitman said.

Then they were flown by C-17 military transport planes to an undisclosed third country. Whitman said it took 37 planeloads to complete that portion of the transfer.

"At this intermediate location the cargo was loaded onto a US-flagged cargo ship, a military sealift container ship, the USS Gopher State," he said.

Whitman said the arrangements for the sale began several months ago, but the transfer itself took "weeks not months."

With the transfer, no yellowcake was known to be left in Iraq, Whitman said.

News of the operation broke over the weekend when Cameco acknowledged the arrival of the uranium shipment at Montreal.

Lyle Khran, a Cameco spokesman, said the company had responded to a bid request made last year by the US government.

"We are satisfied at having been able to remove uranium from one of the most unstable regions of the world, and to have transfered it to a stable region to produce our own electric power," he said on Sunday.

He said the yellowcake would be used at the Blind River and Port Hope nuclear power plants north of Toronto.