But the Air Force contended, as it did last week, that it did not postpone the mission because of Canadian concerns, but due to technical difficulties with the Titan IV rocket.

Thorny issues remain after Canadian and US government and military officials, as well as oil industry representatives, met on Saturday in Halifax, Nova Scotia, to assess the risks of the mission to the Newfoundland offshore, said Shawn Dearn, spokesman for Canadian Public Safety Minister Anne McLellan.

The countries were unable to agree on the environmental impact of a booster and any unspent fuel falling into the ocean, as well as the safety implications should the rocket veer from its intended trajectory, Dearn said.

Meanwhile, Ottawa wants validation of the US Air Force's assessment that the risk of the 10-tonne booster crashing into the 200,000 barrel a day Hibernia platform or any other oil installations in the area is one in 10 trillion, he said.

"It's a pretty minuscule risk, as the minister has said, so at this point federal officials are still in contact with the oil companies as well as the US Air Force and they're looking for answers in some of these areas," he said.

The Air Force plans to use the rocket to launch a classified satellite to be used for US security purposes.

Last Thursday, Canada's offshore oil industry was sent scrambling when regulators ordered an evacuation of the 250 workers from the Hibernia platform, located 315 km (195 miles) southeast of St. John's, Newfoundland, and told operators that output would have to be shut down as a precaution.

Later that day, the Canada-Newfoundland Offshore Petroleum Board lifted the order, and another to tow away a drilling rig in the region contracted to Husky Energy Inc., after the Air Force postponed the launch to Wednesday from Monday. It was pushed back again this weekend.

Canadian officials said their lobbying helped delay the mission, but the Air Force contended it was because the equipment needed to fuel the rocket was late getting to the Cape Canaveral, Florida, launch site.

The latest postponement was because of technical issues as well, said an Air Force official, who questioned why the Canadians were so skittish given the tiny risk.

"We have launched 19 vehicles since 1998 on or near this trajectory without incident," spokeswoman Maj. Karen Finn said. When the Air Force sent a rocket on the exact same path in 1994, "everything landed where we predicted," she said.

Newfoundland Premier Danny Williams told CBC Television that officials are preparing to issue an evacuation order on Thursday, unless there are any changes to the launch plans.

Meanwhile, Hibernia is operating normally, said Alan Jeffers, spokesman for Exxon Mobil Corp., one of the development's six owners.

"We're continuing discussions on both sides of the border with the US and Canadian governments, and really all that we can do at this point is carry on," Jeffers said.